Refactor into modular app structure

Split monolithic files into focused modules:
- app/core: settings, logging, lifecycle
- app/signaling: websocket server, ICE parser, message models
- app/webrtc: peer session, video receiver, frame source
- app/vision: pose landmarker wrapper, model config, pose types
- app/exercises/dead_bug: detector, metrics, rules, state machine, types
- app/rendering: skeleton renderer, status overlay, window display
- app/audio: rep announcer
- app/diagnostics: perf timer, crash handler
- configs: environment-based settings
- tests: unit tests for rules, state machine, ICE parser
- run.py: entry point

app/diagnostics/crash_handler.py

@@ -0,0 +1,11 @@
+from __future__ import annotations
+
+import faulthandler
+from pathlib import Path
+
+
+def enable_crash_handler(log_dir: str | Path) -> None:
+    log_dir = Path(log_dir)
+    log_dir.mkdir(parents=True, exist_ok=True)
+    crash_log = open(log_dir / "posefit-crash.log", "a", buffering=1)
+    faulthandler.enable(file=crash_log, all_threads=True)

app/diagnostics/perf_timer.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+import time
+from contextlib import contextmanager
+
+from loguru import logger
+
+
+class PerfTimer:
+    def __init__(self, name: str = "") -> None:
+        self.name = name
+        self._start = 0.0
+        self._elapsed = 0.0
+
+    def start(self) -> PerfTimer:
+        self._start = time.perf_counter()
+        return self
+
+    def stop(self) -> float:
+        self._elapsed = time.perf_counter() - self._start
+        return self._elapsed
+
+    @property
+    def elapsed_ms(self) -> float:
+        return self._elapsed * 1000
+
+
+@contextmanager
+def measure(name: str = ""):
+    timer = PerfTimer(name).start()
+    yield timer
+    elapsed = timer.stop()
+    if name:
+        logger.debug("{} took {:.1f}ms", name, timer.elapsed_ms)