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

tests/test_dead_bug_state_machine.py

@@ -0,0 +1,42 @@
+from __future__ import annotations
+
+from app.exercises.dead_bug.state_machine import DeadBugStateMachine
+from app.exercises.dead_bug.types import DeadBugMetrics, DeadBugPhase
+
+
+class TestDeadBugStateMachine:
+    def _ready_metrics(self) -> DeadBugMetrics:
+        return DeadBugMetrics(
+            left_arm_extended=False, right_arm_extended=False,
+            left_leg_extended=False, right_leg_extended=False,
+            left_elbow_angle=90, right_elbow_angle=90,
+            left_knee_angle=100, right_knee_angle=100,
+            feedback=[],
+        )
+
+    def _extended_left(self) -> DeadBugMetrics:
+        return DeadBugMetrics(
+            left_arm_extended=True, right_arm_extended=False,
+            left_leg_extended=False, right_leg_extended=True,
+            left_elbow_angle=160, right_elbow_angle=90,
+            left_knee_angle=90, right_knee_angle=160,
+            feedback=[],
+        )
+
+    def test_initial_state(self):
+        sm = DeadBugStateMachine()
+        assert sm.phase == DeadBugPhase.READY
+        assert sm.rep_count == 0
+
+    def test_no_transition_in_ready(self):
+        sm = DeadBugStateMachine()
+        result = sm.update(self._ready_metrics())
+        assert sm.phase == DeadBugPhase.READY
+        assert result.rep_count == 0
+
+    def test_confirm_extension(self):
+        sm = DeadBugStateMachine(extension_confirm_frames=2, reset_confirm_frames=2)
+        sm.update(self._extended_left())
+        assert sm.phase == DeadBugPhase.READY
+        sm.update(self._extended_left())
+        assert sm.phase == DeadBugPhase.EXTENDING