Phase 0 of the WASM metadata migration plan. Exposes typed metadata reads on the WASM NotebookHandle so the frontend can read from its local Automerge doc instead of going through Tauri IPC.

WASM API additions

// Get the full metadata as a typed JSON string
handle.get_metadata_snapshot_json()  // → string | undefined

// Detect runtime from kernelspec/language_info
handle.detect_runtime()              // → "python" | "deno" | undefined

These call through to the already-existing NotebookDoc methods from the notebook-doc shared crate (#562, #566).

Dead code removal

• get_deno_permissions — zero frontend callers
• set_deno_permissions — zero frontend callers

QA

• Existing notebook operations work unchanged (this PR adds API surface, doesn't change any callers yet)
• WASM artifacts build cleanly

WASM binary size: 1104KB → 1194KB (+90KB from pulling in serde_json serialization for the metadata snapshot and the metadata module's types). The notebook-doc crate now includes the metadata types that were previously only in runtimed.

Next: Phase 2 will migrate the read-only Tauri commands (get_notebook_runtime, get_notebook_dependencies, etc.) to use these WASM methods instead of invoke().