fix: address pre-merge review findings — stale identity strings, pyright error, fragile test path, formatting, README accuracy

Required fixes:
- main.py: update module docstring, startup comment, and FastAPI title from
  'tmux-web coordinator' / 'coordinator service' → 'muxplex' (4 locations)
- test_api.py: replace hasattr/BaseRoute pattern with isinstance(r, (APIRoute,
  APIWebSocketRoute)) to satisfy pyright (union-attr error eliminated)

Recommendations applied:
- test_api.py + test_cli.py: auto-formatted with ruff (blank-line normalisation)
- test_cli.py: replace exec(Path('muxplex/__main__.py').read_text()) with
  importlib.util.find_spec() to obtain the absolute path — no longer assumes
  pytest is invoked from a specific working directory
- README.md: fix architecture table — WebSocket proxy moved from ttyd.py
  description to main.py; ttyd.py now correctly described as lifecycle only

All 178 tests pass; python_check clean (0 errors, 0 warnings).

README.md

@@ -171,10 +171,10 @@ muxplex/
     ├── __init__.py
     ├── __main__.py         # `python -m muxplex` entry point
     ├── cli.py              # CLI argument parsing and `install-service` subcommand
-    ├── main.py             # FastAPI app factory and route registration
+    ├── main.py             # FastAPI app: session API, bell hooks, WebSocket proxy to ttyd, static frontend
     ├── sessions.py         # tmux session discovery and snapshot capture
     ├── bells.py            # Bell/activity notification tracking
-    ├── ttyd.py             # ttyd process lifecycle management and WebSocket proxy
+    ├── ttyd.py             # ttyd process lifecycle management (spawn, kill, PID tracking)
     ├── state.py            # Shared in-process state (sessions, bells, ttyd)
     ├── frontend/           # Static frontend assets (served as package data)
     │   ├── index.html