fix(superdeck): release capture resources and prune stale thumbnails

[leoafarias]

Intent

Slide capture built a temporary element, render, and focus tree for every capture and never released it. Thumbnail cleanup ran only inside thumbnail warmup, so a deck that lost slides or became empty kept obsolete handles. Overlapping slide transitions let an earlier timer clear a later transition's state.

This is PR 1 of a three-PR stack. It targets main.

Changes

Capture teardown — SlideCaptureService._releaseCaptureTree rebuilds the root adapter without a child so BuildOwner.finalizeTree unmounts the captured widgets and their State.dispose runs. It then releases the pipeline owner, the render objects the service created, and the focus manager. This runs on success, on the settle limit, and on failure. The focus manager is disposed in a later microtask, because unmounting focus nodes schedules a focus update that must still run against a live manager.

_imageToUint8List disposes the captured image in a finally, so a failed PNG encode no longer leaks it.

Thumbnail pruning — DeckPresentationState prunes obsolete thumbnails from an effect on the slide collection instead of from generateThumbnails. Cleanup now works without warmup and when the deck becomes empty.

Navigation guard — goToSlide carries an operation identifier, so only the latest transition may clear the transitioning state.

Capture dimensions, concurrency limits, readiness tracking, and timeout behaviour are unchanged.

Stacked CI — test, Linux integration, and Chromium/WebKit jobs now run for PRs targeting any branch, including the upper PRs in this stack. All checkout steps use v4 and workflow shell variables pass actionlint.

Impacted packages

• packages/superdeck
• .github/workflows/test.yml

Tests

Eight new regression tests. All eight fail on the pre-change code and pass after it.

• test/src/capture/slide_capture_teardown_test.dart — subtree unmounted after a successful capture, after the settle limit, when a captured widget fails to build, and across a sequence of captures.
• test/src/deck/deck_presentation_state_test.dart — slide removal without warmup, an empty slide collection, an empty warmup list, and a superseded transition.

Commands run

fvm dart run melos run analyze --no-select         # Dart/DCM clean
fvm dart run melos run contracts:check --no-select # SUCCESS
fvm dart run melos run test --no-select            # 2,148 package tests passed on the combined stack

Earlier platform validation reported 39/39 macOS integration tests and 10/10 Chromium/WebKit tests. Those suites were not rerun locally for the workflow-only follow-up; fresh CI now runs them for every stack branch. The browser smoke command used Node 22.18.0.

Follow-up verification: actionlint .github/workflows/test.yml and Dart/DCM analysis passed. New CI runs validate the updated stack.

The follow-up also passed code generation, melos run fix, contract schema checks, and workflow lint, with no generated-file changes.

[github-actions]

Visit the preview URL for this PR (updated for commit 3d78cea):

https://superdeck-dev--pr108-fix-deck-runtime-lif-08719o64.web.app

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