Cancel FFmpeg library work when the dialog closes

[Blackspirits]

Purpose

Dependent lifecycle follow-up to #135 ("Install verified FFmpeg libraries transactionally").

#134 verifies the downloaded archive and #135 makes installation rollback-safe. This tranche closes the remaining dialog-lifetime gap: closing the FFmpeg libraries window through the title-bar X must cancel the same work as the explicit Cancel/Escape paths and must not leave a partial ZIP in the data folder.

Current upstream SubtitleEdit/subtitleedit main was rechecked at d6917286e355976da953d0ee22208b4bf9461699; its latest change is unrelated to this downloader.

Finding fixed

DownloadFfmpegLibsViewModel implements IClosingCleanup, but OnClosingCleanup() previously only stopped/disposed its polling timer.

Closing the window with the title-bar X could therefore leave the HTTP/hash task running, leave a partial/full ZIP in Se.DataFolder, and allow already-started unpack/install work to outlive the window.

Fix

• make _done visible across timer/UI threads;
• centralize cancellation in CancelPendingWork();
• Cancel/Escape and window cleanup cancel the same token used by download/hash and Install verified FFmpeg libraries transactionally #135 installation;
• make cleanup idempotent through an interlocked gate;
• stop the indeterminate progress helper during cancellation;
• refuse to start it after cancellation;
• observe the download task asynchronously and delete the temp ZIP once its file handle is released;
• retain Install verified FFmpeg libraries transactionally #135 rollback semantics if close lands during installation.

Normal successful close remains safe and idempotent.

Regression coverage

Tests pin that:

• OnClosingCleanup() cancels a still-running service token;
• temp cleanup waits for a pending task before deleting the ZIP;
• faulted tasks are observed during cleanup and still have their ZIP removed;
• the blocking wait in the lifecycle regression participates in xUnit cancellation, so this tranche adds no compiler/analyzer warning.

Scope / branch state

• dependency/base PR: Install verified FFmpeg libraries transactionally #135
• base HEAD: dd7e85e4319154aaf3105f85e745509ea902c4dc
• HEAD: eed96e1a1c5f3c57411293fdd3ee56316c385ef0
• 1 commit
• 2 files
• +142 / -11
• current upstream main checked: d6917286e355976da953d0ee22208b4bf9461699
• audit draft only; no merge/promotion intended

AI assistance: ChatGPT was used for adversarial window-close/timer/task lifecycle review, cancellation race modelling and temporary-file cleanup regression design.

Final CI

Authoritative run: SubtitleEdit#273 on eed96e1a1c5f3c57411293fdd3ee56316c385ef0

• SeConv: 488 passed / 2 skipped / 0 failed
• LibUiLogic: 905 passed / 0 skipped / 0 failed
• LibSE: 2017 passed / 0 skipped / 0 failed
• UI: 5240 passed / 9 skipped / 0 failed
• build: 0 errors / 7 pre-existing warnings
• retry step skipped
• failure-artifact upload skipped

Final adversarial review was performed on this exact HEAD after CI; no blocker found.

[Blackspirits]

Final adversarial review on eed96e1a1c5f3c57411293fdd3ee56316c385ef0 after CI SubtitleEdit#273: no blocker found. Verified X/Cancel/Escape converge on the same cancellation token; an already-running timer callback can no longer outlive the dialog without observing cancellation; temporary ZIP cleanup waits for the download task to release the file; #135 supplies rollback if cancellation lands during install. The earlier xUnit1051 warning was removed before this final run. No merge/promotion performed.