feat(config): map custom file extensions to languages via codegraph.json (#906) (#955)

The extension → language table was hardcoded, so a codebase using a
non-standard extension for a supported language (e.g. `.dota_lua` for Lua)
had those files silently skipped — no way to opt them in short of patching
the source.

Add an opt-in, project-scoped `codegraph.json` at the repo root:

    { "extensions": { ".dota_lua": "lua", ".tpl": "php" } }

Mappings merge on top of the built-in defaults and take precedence (so a
built-in can be re-pointed, e.g. `.h` → `cpp`). Absent or malformed config
is the zero-config default — byte-identical to prior behavior; an invalid
target language or unparseable file is warned-and-skipped, never fatal.

Implementation:
- New `src/project-config.ts` — `loadExtensionOverrides(rootDir)`, validated
  against `isLanguageSupported`, mtime-cached per root.
- `detectLanguage` / `isSourceFile` gain an optional `overrides` arg
  (omitting it is the existing behavior).
- Overrides threaded per-operation through every extraction call site
  (scan/walk gates, git change-detection, grammar selection, extraction,
  the file watcher), resolved from the project root — no process-global
  state, so the multi-project daemon stays isolated. The parse worker
  receives the resolved language in its message.

Tests: 13 new cases (unit, loader validation/normalization/caching, and a
full-index integration proving a custom-extension file is extracted while
the zero-config path indexes nothing). Worker path smoke-tested via the
built CLI.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -30,6 +30,7 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 - Java `static final` constants, C# `const` / `static readonly` constants, Scala `object` vals, and Kotlin top-level / `object` / `companion object` `val`s are now classified as constants rather than generic fields, so they participate in the constant-reader impact analysis above — change a `public static final` table, a `const string`, a Scala `object Config { val Timeout = … }`, or a Kotlin `companion object { const val … }` and the methods that read it now show up as affected. (Per-object Java `final` / C# `readonly` / Scala & Kotlin `class` instance properties are unchanged.) Kotlin constants were previously not indexed as their own symbols at all, so they now also appear in `codegraph search`.
 - Swift top-level `let`s and `static let` constants (including those namespaced in an `enum`/`struct`, the common Swift pattern) are now indexed as constants and participate in the constant-reader impact analysis above — change a `static let defaultRetryLimit` or an `enum Constants { static let … }` and the same-file code that reads it shows up as affected. Computed properties and per-instance `let`s are not treated as constants.
 - Dart top-level `const`/`final` and class `static const`/`static final` constants are now indexed as constants and participate in the constant-reader impact analysis above. Instance fields, `var`s, and locals are not treated as constants. (Generated Dart code with the standard `.g.dart`/`.freezed.dart`/`.pb.dart` suffixes is already skipped.)
+- You can now teach CodeGraph about custom file extensions. Drop a `codegraph.json` at your repo root with an `extensions` map — `{ "extensions": { ".dota_lua": "lua", ".tpl": "php" } }` — and files with those extensions get indexed under the language you name, instead of being silently skipped because the extension wasn't one of the built-in defaults. It's opt-in and committed alongside your code so the whole team shares it, your mappings layer on top of the built-ins and win on conflict (you can even re-point a built-in, e.g. `.h` → `cpp`), and a typo'd language or a malformed config is warned about and skipped rather than breaking indexing. Projects without a `codegraph.json` behave exactly as before. (#906)
 
 ### Fixes