Add rust-analyzer integration with find_rust_symbol MCP tool

[hodgesds]

Adds rust-analyzer-backed symbol lookup to semcode. When the indexed repository
is a Rust project (has a Cargo.toml), semcode spawns rust-analyzer as a
background LSP process and exposes a new find_rust_symbol MCP tool that answers
workspace/symbol queries using rust-analyzer's exact type inference — more
precise than the tree-sitter index for Rust code.

What's included

Async JSON-RPC rust-analyzer client (src/rust_analyzer.rs)

• Spawns rust-analyzer over stdio with LSP Content-Length framing, dedicated
  sender/reader tasks, and request routing via a oneshot map.
• Spawned with kill_on_drop and the handle retained, so the process is
  terminated with the client instead of being orphaned.
• 30s per-request timeout so a wedged/dead server can't hang callers.
• Returns the unwrapped JSON-RPC result to callers.

DatabaseManager integration (src/database/connection.rs)

• attach_rust_analyzer() starts the client only when a Cargo.toml is present;
  the query, LSP, and MCP binaries attach on startup (MCP attaches in the
  background to avoid blocking init).

find_rust_symbol MCP tool (src/bin/semcode-mcp.rs)

• New tool in the code_lookup category with schema and dispatch.
• Empty results return an "indexing may still be in progress — retry" hint,
  since workspace/symbol returns nothing until rust-analyzer finishes indexing.

Async workdir index (src/workdir.rs, query.rs, semcode-lsp.rs)

• WorkdirIndex::build[_incremental] and the workdir-refresh paths are now async
  to support the async client.

Testing

• cargo fmt --check, cargo clippy --all-targets -- -D warnings: clean.
• cargo test: all passing, including new unit tests for JSON-RPC framing and
  request/notification serialization, and the updated MCP tool-count tests.