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.

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)