fix(dismissable-layer): prevent escape from dismissing underlying layer - #4145

Description

Fixes #4143.

When a new layer registers, the previous top layer can retain its keydown listener until React processes the layer update. An Escape press in that window could therefore be handled by the stale listener and dismiss the layer underneath.

This change keeps the render-time listener optimization, but verifies the current layer stack again at event time before invoking the Escape handlers. It also adds a deterministic regression test that dispatches Escape during child-layer registration and a patch changeset.

The synchronization commit 4493392536929a77864b3fca6cf29d34ed595be9 preserves the upstream IME Escape guard before the live layer-stack guard. The regression test merged without conflict and the original patch changeset remains.

Validation

On merge commit 44933925, with frozen-lockfile dependencies:

• Dismissable Layer official unit suite: 36 passed.
• Eight related consumer suites: 281 passed, 4 existing TODO, 0 failed (dialog, alert-dialog, context-menu, dropdown-menu, hover-card, popover, select and tooltip).
• Package lint and typecheck passed; the same-dependency main typecheck also passed.
• Canonical formatting checks for the two changed TSX files and contribution git diff --check passed.
• The full repository suite, browser tests and build were not rerun on this merge. The previous 981-test/full-build validation belongs to the earlier head and is not reused as a check for this commit.

Upstream workflows currently require maintainer approval before execution; local validation does not establish that upstream CI passed.

AI-assisted follow-up validation and summary (GPT-6).