feat(config): add submit_id platform override for language selection

Problem: The submission language ID is hardcoded per platform in
`language_ids.py` (e.g. CF `"89"` = GNU G++17 7.3.0). Users have no
way to select a different version like G++20 or C++23.

Solution: Add `submit_id?: string` to `CpPlatformOverrides` and
`CpLanguage`. When set, `submit.lua` passes the resolved `submit_id`
directly to the scraper instead of the generic language key. The
existing `get_language_id() or args[4]` fallback in `base.py` handles
pre-resolved IDs without any Python changes.

lua/cp/config.lua

@@ -8,6 +8,7 @@
 ---@field extension string
 ---@field commands CpLangCommands
 ---@field template? string
+---@field submit_id? string
 
 ---@class CpTemplatesConfig
 ---@field cursor_marker? string
@@ -16,6 +17,7 @@
 ---@field extension? string
 ---@field commands? CpLangCommands
 ---@field template? string
+---@field submit_id? string
 
 ---@class CpPlatform
 ---@field enabled_languages string[]
@@ -293,6 +295,9 @@ local function merge_lang(base, ov)
   if ov.template then
     out.template = ov.template
   end
+  if ov.submit_id then
+    out.submit_id = ov.submit_id
+  end
   return out
 end