Fix ECG SVG repaint on Mobile WebKit (remove paint containment; add regression test)

docs/branch-review-records/3dc49fdd6e24d09e3e6830eb12934c340fc3a9008650b6c058d34bd4d87a2afe.record.md

@@ -0,0 +1 @@
+| 2026-08-15 | codex/fix-ecg-animation-on-mobile-devices | 4c789e7449613bbae0701610414214586fee177a | ECG SVG repaint on Mobile WebKit | Confirmed the focused WebKit paint-containment correction; no additional P0-P2 findings. Merged the latest required base. | git diff --check; ledger inbox/outstanding-issues/branch-ledger/discipline guards; direct ECG CSS regression assertion; focused Vitest attempted but unavailable because the isolated worktree has no node_modules |

src/app/globals.css

@@ -3183,10 +3183,6 @@ td,
  * those unlayered classes, check the class body first; the class wins.
  */
 @layer components {
-  .answer-activity-trace {
-    contain: paint;
-  }
-
   .answer-activity-trace__sweep {
     animation: var(--animate-answer-ecg);
   }

tests/answer-activity-trace-css.test.ts

@@ -24,6 +24,10 @@ function keyframes(name: string) {
 }
 
 describe("answer activity trace CSS", () => {
+  it("does not paint-contain the animated SVG on WebKit", () => {
+    expect(globalsCss).not.toMatch(/\.answer-activity-trace\s*{[^}]*contain:\s*paint;/s);
+  });
+
   it("cycles through the positive dash-offset equivalent for WebKit", () => {
     const sweep = keyframes("answer-ecg-sweep");