Fix main-document switching end to end; stable project-independent MCP pathHEADmain

Main document: the app never called Overleaf's official settings endpoint,
so "Set as main document" only changed renderer state and agents had no way
to switch at all — compiles kept using the old root. Now:
- Right-click Set as main and the new set_main_file MCP tool both POST
  /project/:id/settings {rootDocId} (web parity, persists project-wide)
- Handle the rootDocUpdated broadcast: update the project snapshot, the
  renderer state, and rewrite CLAUDE.md/AGENTS.md — the guide's stale
  "Main file" line was teaching agents to compile the old root via
  main_file overrides
- Compile root resolution uses the sync bridge's live maps, so docs
  created mid-session resolve correctly (agent guide too)

MCP server location: Codex stores its MCP registration globally, and the
per-project path went stale when the project temp dir was cleaned up
("connection closed" on handshake). The bundled server now installs to
~/.lattex/lattex-mcp.mjs (project-independent, refreshed on connect; the
esbuild bundle since the raw source can't resolve node_modules from there)
and resolves the project at runtime: $LATTEX_PROJECT_DIR, cwd + ancestors,
legacy script location, then single live project in tmpdir. Register once,
works for every project.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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