lsp: add helix-style language server support

[aplio]

What

Adds Helix-style LSP support to the native TUI, including:

• explicit language IDs, root markers, ordered multi-server routing, and feature filters
• Java, JavaScript, JSX, TypeScript, and TSX defaults
• capability-aware lifecycle, sync, timeout, stale-response, and shutdown handling
• completion/resolve/snippet sessions, hover, signature help, navigation, diagnostics, formatting, rename, code actions, symbols, inlay hints, document highlights, colors, and links
• fake stdio coverage and opt-in real-server tests for TypeScript Language Server and Eclipse JDT LS

The inventory, implemented surface, and remaining gaps are documented in docs/lsp-parity.md.

Why

Gargo previously had syntax highlighting for Java and JavaScript-family files, but its LSP client only exposed hover, definition, references, and push diagnostics. Java/JS servers required manual configuration, JSX/TSX wire IDs were incompatible with common servers, roots were Git-only, and server requests/capabilities were largely ignored.

This makes the common language-server path useful while keeping unsupported behavior explicit. It does not claim complete Helix parity; remaining work includes virtual jdt:// documents, watched-file registration, full TextMate snippet variables/transforms, cancellation, and several workspace features.

Completion interaction

Tree-sitter is not an insertion-completion provider in Gargo; it supplies highlighting and symbol/index navigation. The existing insertion fallback is Markdown link-path completion.

This change verifies that LSP and Markdown candidates are mutually consistent, snippet Tab navigation has priority, modal palettes dismiss pending/stale results, and typing plus Ctrl-Space continue through the overlay stack correctly.

Real-server verification

• Node.js v24.13.1, TypeScript Language Server 5.3.0, TypeScript 6.0.3
  • JSX/TSX IDs, completion, hover, diagnostics, cross-file definition, shutdown
• Temurin 21.0.12+8, Eclipse JDT LS 1.60.0
  • diagnostics, completion, hover, cross-file definition, shutdown

Exact pinned installation and opt-in test commands are in tests/README.md. No downloaded runtime, node_modules, JDK, or JDT LS artifact is committed.

Checks

• cargo fmt --all -- --check
• git diff --check
• cargo check --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --lib lsp -- --test-threads=1 (142 passed, 1 subprocess entrypoint ignored)
• full workspace suite with isolated XDG_DATA_HOME (1,378 lib tests passed; all integration tests passed)
• real TypeScript Language Server E2E
• real Eclipse JDT LS E2E