feat: add Phase 1 Language Host with tree-sitter features

[j4rviscmd]

Summary

Phase 1 of the native (Rust) Language Host: tree-sitter-backed documentSymbol and foldingRange for the configured languages, replacing the Phase 0 echo probe.

Changes

• Wire protocol (languageHostProtocol.ts): unified the frame to 8 bytes [reqId(4)][len(4)][payload] across renderer, electron-main relay, and Rust. Added a request-id multiplexer — notifications (reqId=0, no response) for document sync, requests (reqId>0, response awaited) for language features.
• Document sync (documentSync.ts): didOpen / didChange / didClose over notifications. Phase 1 sends the full text on every change (full-text replace) to avoid an incremental range-merge engine.
• Language features (languageFeatures.ts): documentSymbol and foldingRange providers registered with ILanguageFeaturesService for the configured languages.
• electron-main relay (languageHostServerService.ts): simplified to a transparent 8-byte-frame pass-through. The Phase 0 relay's 4-byte length header misread the renderer's reqId as the payload length, which broke the path.
• Rust host (main.rs): tree-sitter + tree-sitter-typescript parse the document and emit symbols (function / class / method / interface / enum / type) and foldable block ranges.
• Removed the Phase 0 echo probe; the documentSymbol / foldingRange path now carries the wire validation.

How to verify

1. `cd rust && cargo build` (binary lands at `rust/target/debug/coderm-language-host`)
2. `npm run watch` + `./scripts/code.sh`
3. Enable `coderm.languageHost.enabled` and set `coderm.languageHost.languages` (e.g. `["typescript", "tsx"]`)
4. Open a `.ts` file → check Outline (`Cmd+Shift+O`) and folding

Default remains off; with an empty `languages` array the feature is inert.

Notes

• Pinned to `tree-sitter` / `tree-sitter-typescript` 0.23 (the grammar's 0.22 is not on crates.io).
• Known Phase 1 limitations (documented inline): full-text replace on change, flat symbol list (VS Code re-hierarchizes by range overlap), 1 MiB max payload, and a +1 column conversion that is ASCII-only (non-ASCII reconciliation is deferred to Phase 2).