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/vegas.json

@@ -0,0 +1,26 @@
+{
+  "screens": [
+    [0.0, "<vegas>", 30.008, "duration", 3751, null],
+    [30.008, "<vegas>", 30.007, "duration", 3751, null],
+    [60.015, "<vegas>", 10.24, "vegas-live", 1280, null],
+    [70.255, "clock", 20.0, "duration", 20, false],
+    [90.255, "sports_live", 20.0, "display-false", 11, true],
+    [110.255, "<vegas>", 30.008, "duration", 3751, null],
+    [140.263, "<vegas>", 10.0, "on-demand-start", 1250, null],
+    [150.263, "clock", 20.0, "duration", 20, true],
+    [170.263, "clock", 5.0, "on-demand-expired", 5, true],
+    [175.263, "<vegas>", 24.959, "vegas-interrupt", 3120, null],
+    [200.222, "clock", 20.0, "duration", 20, true],
+    [220.222, "<vegas>", 30.008, "duration", 3751, null],
+    [250.23, "<vegas>", 9.77, "horizon", 1222, null]
+  ],
+  "events": [
+    [70.255, "vegas-live"],
+    [150.0, "request", "start:v1"],
+    [150.263, "on-demand-start", "clock"],
+    [150.263, "vegas-interrupt"],
+    [175.263, "on-demand-expired"],
+    [200.0, "wifi-file", "Connected to HomeNet"],
+    [200.222, "vegas-interrupt"]
+  ]
+}