Final runtime proof at 2fe77f4048 (production change includes forced-close bound)
fs-14 Linux x86_64; isolated worktree vp run dev; web8564/server16604;
product-native headless server preview at1280x800, deviceScaleFactor2;
no device emulation. Same fixture extended with attempt labels, ready beacon,
and optional guard or freeze-on-button-click. Raw final-events.json attached.

Plain incognito (isolated context), no activation:
09:59:57.907 ready; close returned Success10:00:03.465.
Only pagehide receipt10:00:03.434. beforeunload/visibilitychange receipts
missing in this attempt. This is a delivery limitation; receipt alone cannot
prove the corresponding event did not dispatch.

Shared-profile activated guard:
10:00:16.531 ready. Authenticated production PreviewStreamClient granted control,
mouse down/up sent at A(20,18), both true. preventDefault/returnValue guard.
Close Success10:01:05.780; beforeunload10:01:05.695,
pagehide10:01:05.745, visibilitychange10:01:05.755.

Incognito isolated activated guard:
10:01:05.928 ready; same production stream mouse activation(20,18), sends true.
Close Success10:01:37.232; beforeunload10:01:37.111,
pagehide10:01:37.129, visibilitychange10:01:37.140.

Frozen isolated page:
10:02:15.444 ready; click A starts for(;;){} after a freeze-start beacon.
10:02:50.626 freeze-start receipt. Renderer pid573090 (found by ancestry from
dev-server listener pid557465) had12 CPU ticks before;633ticks at close.
10:02:58.845 close returned Success, duration85ms. Renderer still existed
immediately afterward. After a6second runtime wait, no renderer descendants
of that dev server remained. No kill command was used. Forced-close fallback
stops a renderer that cannot process graceful beforeunload.

Repeated plain incognito (isolated context), no activation:
10:03:22.082 ready. Close Success10:03:31.616.
beforeunload10:03:31.557, visibilitychange10:03:31.570,
pagehide10:03:31.572. All3 delivered on this repetition.

Production close bounded5s; normal close clears timer. 26focused tests pass.
What else changed: no render code; close lifecycle and cleanup only. Visual
matrix0/0; no screenshots/recordings applicable. Native desktop adapter unchanged,
not exercised. HTTP receipt order is not event dispatch order. Do not claim
beacon delivery is universal: the first incognito attempt above was incomplete.