feat(config): add language version selection with setup-time validation

Problem: users cannot choose which C++ standard (e.g. C++17 vs C++20
vs C++23) to submit with — the plugin hardcodes a single platform ID
per language with no discoverability or override mechanism.

Solution: add `LANGUAGE_VERSIONS` and `DEFAULT_VERSIONS` tables in
`constants.lua` as the single source of truth. `config.lua` gains
`version` and `submit_id` fields on `CpLanguage`/`CpPlatformOverrides`,
validated at setup time. `submit.lua` resolves the effective config to
a platform ID (priority: `submit_id` > `version` > default). Helpdocs
document the new `*cp-submit-language*` section with per-platform
available versions and the `submit_id` escape hatch. Tests cover
`LANGUAGE_IDS` completeness and `get_language_id` lookups.

doc/cp.nvim.txt

@@ -24,15 +24,16 @@ CONTENTS                                                        *cp-contents*
  16. Race .......................................................... |cp-race|
  17. Credentials ............................................ |cp-credentials|
  18. Submit ...................................................... |cp-submit|
- 19. Open ......................................................... |cp-open|
- 20. ANSI Colors ................................................... |cp-ansi|
- 21. Highlight Groups ........................................ |cp-highlights|
- 22. Terminal Colors .................................... |cp-terminal-colors|
- 23. Highlight Customization .......................... |cp-highlight-custom|
- 24. Helpers .................................................... |cp-helpers|
- 25. Statusline Integration .................................. |cp-statusline|
- 26. Panel Keymaps .......................................... |cp-panel-keys|
- 27. File Structure ................................................ |cp-files|
+ 19. Submit Language Versions ......................... |cp-submit-language|
+ 20. Open ......................................................... |cp-open|
+ 21. ANSI Colors ................................................... |cp-ansi|
+ 22. Highlight Groups ........................................ |cp-highlights|
+ 23. Terminal Colors .................................... |cp-terminal-colors|
+ 24. Highlight Customization .......................... |cp-highlight-custom|
+ 25. Helpers .................................................... |cp-helpers|
+ 26. Statusline Integration .................................. |cp-statusline|
+ 27. Panel Keymaps .......................................... |cp-panel-keys|
+ 28. File Structure ................................................ |cp-files|
  28. Health Check ................................................ |cp-health|
 
 ==============================================================================
@@ -1032,7 +1033,58 @@ Submit the current solution to the online judge.
 
         Platform support:
             AtCoder     Fully implemented.
-            Others      Not yet implemented.
+            Codeforces  Fully implemented.
+            CSES        Fully implemented.
+            Kattis      Fully implemented.
+            USACO       Fully implemented.
+            CodeChef    Not yet implemented.
+
+        See |cp-submit-language| for configuring the language version
+        (e.g. C++20 instead of C++17).
+
+==============================================================================
+SUBMIT LANGUAGE VERSIONS                              *cp-submit-language*
+
+When submitting, cp.nvim selects a language version for the platform.
+The default is C++17 for cpp and Python 3 for python.
+
+Configuring a version ~
+
+Set {version} globally or per-platform:
+>lua
+    languages = {
+      cpp = { version = "c++20", ... },
+    },
+    platforms = {
+      atcoder = {
+        overrides = { cpp = { version = "c++23" } },
+      },
+    },
+<
+Available versions per platform ~
+
+  Platform    cpp              python
+  AtCoder     c++23            python3
+  Codeforces  c++17            python3
+  CSES        c++17            python3
+  Kattis      c++17/20/23      python3
+  USACO       c++17/20/23      python3
+  CodeChef    c++17            python3
+
+Using a raw platform ID ~
+
+If your preferred version is not listed, you can bypass version
+lookup by setting {submit_id} to the raw platform language ID:
+>lua
+    platforms = {
+      codeforces = {
+        overrides = { cpp = { submit_id = "91" } },
+      },
+    },
+<
+To find the raw ID, open the platform's submit page in your browser,
+inspect the language dropdown, and copy the <option value="..."> for
+your desired language.
 
 ==============================================================================
 OPEN                                                                 *cp-open*

lua/cp/config.lua

@@ -8,6 +8,8 @@
 ---@field extension string
 ---@field commands CpLangCommands
 ---@field template? string
+---@field version? string
+---@field submit_id? string
 
 ---@class CpTemplatesConfig
 ---@field cursor_marker? string
@@ -16,6 +18,8 @@
 ---@field extension? string
 ---@field commands? CpLangCommands
 ---@field template? string
+---@field version? string
+---@field submit_id? string
 
 ---@class CpPlatform
 ---@field enabled_languages string[]
@@ -293,6 +297,12 @@ local function merge_lang(base, ov)
   if ov.template then
     out.template = ov.template
   end
+  if ov.version then
+    out.version = ov.version
+  end
+  if ov.submit_id then
+    out.submit_id = ov.submit_id
+  end
   return out
 end
 
@@ -323,6 +333,23 @@ local function build_runtime(cfg)
       validate_language(lid, base)
       local eff = merge_lang(base, p.overrides and p.overrides[lid] or nil)
       validate_language(lid, eff)
+      if eff.version then
+        local normalized = eff.version:lower():gsub('%s+', '')
+        local versions = (constants.LANGUAGE_VERSIONS[plat] or {})[lid]
+        if not versions or not versions[normalized] then
+          local avail = versions and vim.tbl_keys(versions) or {}
+          table.sort(avail)
+          error(
+            ("[cp.nvim] Unknown version '%s' for %s on %s. Available: [%s]. See :help cp-submit-language"):format(
+              eff.version,
+              lid,
+              plat,
+              table.concat(avail, ', ')
+            )
+          )
+        end
+        eff.version = normalized
+      end
       cfg.runtime.effective[plat][lid] = eff
     end
   end

lua/cp/constants.lua

@@ -74,4 +74,21 @@ M.signal_codes = {
   [143] = 'SIGTERM',
 }
 
+M.LANGUAGE_VERSIONS = {
+  atcoder = { cpp = { ['c++23'] = '6017' }, python = { python3 = '6082' } },
+  codeforces = { cpp = { ['c++17'] = '89' }, python = { python3 = '70' } },
+  cses = { cpp = { ['c++17'] = 'C++17' }, python = { python3 = 'Python3' } },
+  kattis = {
+    cpp = { ['c++17'] = 'C++', ['c++20'] = 'C++', ['c++23'] = 'C++' },
+    python = { python3 = 'Python 3' },
+  },
+  usaco = {
+    cpp = { ['c++17'] = 'cpp', ['c++20'] = 'cpp', ['c++23'] = 'cpp' },
+    python = { python3 = 'python' },
+  },
+  codechef = { cpp = { ['c++17'] = 'C++ 17' }, python = { python3 = 'Python 3' } },
+}
+
+M.DEFAULT_VERSIONS = { cpp = 'c++17', python = 'python3' }
+
 return M

lua/cp/submit.lua

@@ -1,6 +1,8 @@
 local M = {}
 
 local cache = require('cp.cache')
+local config = require('cp.config')
+local constants = require('cp.constants')
 local logger = require('cp.log')
 local state = require('cp.state')
 
@@ -56,6 +58,24 @@ function M.submit(opts)
   end
   source_file = vim.fn.fnamemodify(source_file, ':p')
 
+  local submit_language = language
+  local cfg = config.get_config()
+  local plat_effective = cfg.runtime and cfg.runtime.effective and cfg.runtime.effective[platform]
+  local eff = plat_effective and plat_effective[language]
+  if eff then
+    if eff.submit_id then
+      submit_language = eff.submit_id
+    else
+      local ver = eff.version or constants.DEFAULT_VERSIONS[language]
+      if ver then
+        local versions = (constants.LANGUAGE_VERSIONS[platform] or {})[language]
+        if versions and versions[ver] then
+          submit_language = versions[ver]
+        end
+      end
+    end
+  end
+
   prompt_credentials(platform, function(creds)
     vim.cmd.update()
     vim.notify('[cp.nvim] Submitting...', vim.log.levels.INFO)
@@ -64,7 +84,7 @@ function M.submit(opts)
       platform,
       contest_id,
       problem_id,
-      language,
+      submit_language,
       source_file,
       creds,
       function(ev)

tests/test_scrapers.py

@@ -1,5 +1,6 @@
 import pytest
 
+from scrapers.language_ids import LANGUAGE_IDS, get_language_id
 from scrapers.models import (
     ContestListResult,
     MetadataResult,
@@ -137,3 +138,28 @@ def test_scraper_metadata_error(run_scraper_offline, scraper, contest_id):
     assert objs
     assert objs[-1].get("success") is False
     assert objs[-1].get("error")
+
+
+EXPECTED_PLATFORMS = {"atcoder", "codeforces", "cses", "usaco", "kattis", "codechef"}
+EXPECTED_LANGUAGES = {"cpp", "python"}
+
+
+def test_language_ids_coverage():
+    assert set(LANGUAGE_IDS.keys()) == EXPECTED_PLATFORMS
+    for platform, langs in LANGUAGE_IDS.items():
+        assert set(langs.keys()) == EXPECTED_LANGUAGES, f"{platform} missing languages"
+        for lang, lid in langs.items():
+            assert isinstance(lid, str) and lid, f"{platform}/{lang} empty ID"
+
+
+@pytest.mark.parametrize("platform", EXPECTED_PLATFORMS)
+@pytest.mark.parametrize("language", EXPECTED_LANGUAGES)
+def test_get_language_id(platform, language):
+    result = get_language_id(platform, language)
+    assert result is not None, f"No ID for {platform}/{language}"
+    assert result == LANGUAGE_IDS[platform][language]
+
+
+def test_get_language_id_unknown():
+    assert get_language_id("nonexistent", "cpp") is None
+    assert get_language_id("codeforces", "rust") is None