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).

muxplex/main.py

@@ -1,7 +1,7 @@
 """
-FastAPI coordinator application for tmux-web.
+muxplex — FastAPI application for the tmux session dashboard.
 
-Entry point for the coordinator service. Exposes:
+Entry point for the muxplex server. Exposes:
     GET /health  →  {"status": "ok"}
 
 Background poll loop reconciles tmux session state every POLL_INTERVAL seconds.
@@ -138,7 +138,7 @@ async def _poll_loop() -> None:
 async def lifespan(app: FastAPI):
     global _poll_task
 
-    # Startup: kill any orphaned ttyd from a previous coordinator run, then
+    # Startup: kill any orphaned ttyd from a previous muxplex run, then
     # start the background poll loop.
     await kill_orphan_ttyd()
     _poll_task = asyncio.create_task(_poll_loop())
@@ -170,7 +170,7 @@ async def lifespan(app: FastAPI):
 # App
 # ---------------------------------------------------------------------------
 
-app = FastAPI(title="tmux-web coordinator", version="0.1.0", lifespan=lifespan)
+app = FastAPI(title="muxplex", version="0.1.0", lifespan=lifespan)
 
 
 # ---------------------------------------------------------------------------