feat: add Phase 2 hover provider + Phase 1.5 robustness

[j4rviscmd]

Summary

Phase 2 of the native (Rust) Language Host: tree-sitter-backed hover provider (function/method/class/interface/type/typed-variable signatures + JSDoc), plus Phase 1.5 robustness, plus a SymbolKind off-by-one fix for the Phase 1 documentSymbol icons.

Phase 2 — Hover

• Message::Hover { uri, line, column } → HoverResponse { signature, documentation, range }
• UTF-16 → UTF-8 column reconciliation for hover positions (renderer Position.column is UTF-16 code units; tree-sitter Point.column is a UTF-8 byte offset). Phase 1 symbol/fold stay byte-column + 1 with the retained TODO.
• Signature via source-slice of the declaration header (keeps async / generics / params / return-type verbatim — no reconstruction); JSDoc collected from leading /** comment siblings.
• Name-token gate (hover fires only on the identifier, not the body); variable_declarator / public_field_definition require a type annotation.
• CodermHoverProvider registered for the configured languages; renderer wraps signature in a ```typescript MarkdownString and renders JSDoc as a second markdown string.

Phase 1.5 — Robustness

• Reject all pending requests on MessagePort close (no 30s timeout wait).
• onDidChangeLanguage re-sync: document/open re-sent with the new languageId.

Fix

• SymbolKind off-by-one: documentSymbol kind values now match VS Code's SymbolKind enum (Class=4, Method=5, Enum=9, Interface=10, Function=11, TypeParameter=25), not LSP's off-by-one numbers. Outline icons now render correctly.

How to verify

1. `cd rust && cargo build`
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:
   • Outline icons (`Cmd+Shift+O`) render correctly per kind
   • Hover a function/class/interface name → signature + JSDoc; hover inside a body → nothing; `const x: number` shows, `const x = 1` doesn't
   • Change language mode → host re-syncs

Verified end-to-end via Playwright CDP (hover signature + JSDoc render, source-slice keeps generics).