refactor(display): run() stage 1 - golden traces and extracted helpers, no behaviour change (#704)

Adds golden trace tests for DisplayController.run() (test/test_run_loop_golden.py on a fake clock with fake plugins, 15 scenarios, fixtures in test/fixtures/run_loop_golden/) and moves twelve blocks of run() into named helpers (_dispatch_first_frame, _resolve_durations, _resolve_active_mode, _needs_high_fps, _advance_after_screen and others) with the traces identical before and after. docs/RUN_LOOP_REDESIGN.md describes the target structure. Hardware-checked on hdpi: Vegas late-frame rate unchanged in an ABBA A/B.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/fixtures/run_loop_golden/follower.json

@@ -0,0 +1,10 @@
+{
+  "screens": [
+    [0.0, "clock", 20.0, "duration", 20, false],
+    [20.0, "weather", 20.0, "duration", 20, true],
+    [40.0, "<follower>", 10.017, "duration", 601, null],
+    [50.017, "clock", 20.0, "duration", 20, true],
+    [70.017, "weather", 9.983, "horizon", 10, true]
+  ],
+  "events": []
+}