fix(desktop): stop hoarding installed runtime archives

[thcp]

Every runtime pack a user ever installed stayed on disk at full size.

Closes #356

What was wrong

extract_runtime_pack deletes its temp directory and the runtime it displaced, but never the archive it extracted from. Measured on a machine running 0.8.0-alpha.13:

165 MB  StemDeck-runtime-macOS-arm64.tar.zst   18 Jul
 26 MB  ffmpeg-macos.zip                        4 Jun
 26 MB  ffprobe-macos.zip                       4 Jun

207 MB of a 1.3 GB app-data directory, none of it reachable again.

What changed

The archive is removed once its runtime is installed. A version change additionally sweeps whatever the previous build left in downloads/, plus a runtime.tmp or runtime.old from a swap that did not finish -- that one can be a second full runtime, around 900 MB.

The expected archive is spared only while it is still needed. Its filename carries no version, so once the matching runtime is installed the pack is dead weight that would otherwise sit there looking current forever. runtime_is_current answers that question from the install manifest. Keeping it while an install is pending also means an update that already downloaded the pack does not fetch it twice, and a partial download is not deleted from under the request writing it.

Installing a stale pack was never a risk, incidentally: the manifest SHA256 is verified before extraction.

What is deliberately not swept

settings.json lives in this directory and now holds jobs_dir, the stems location from #354. Removing it would send a user who moved their library to an external disk back to the default folder, opening to an empty app with their stems stranded at the old path. There is a test that asserts it survives, alongside config.json, runtime/, ffmpeg/ and models/.

Those last three stay for a plainer reason: re-downloading ~350 MB on every update to produce an identical folder costs more than the disk it frees. The runtime already refreshes itself properly when its version changes, on all three platforms.

Testing

28 Rust tests, 9 new, covering: a stale archive removed while the expected one survives; nothing outside downloads/ touched; an unfinished swap cleaned while the live runtime stays; a missing folder tolerated; and the three states of runtime_is_current (matching, older, half-installed).

Best-effort throughout -- a file that will not delete is worth a log line, never a failed launch. cargo clippy -- -D warnings shows no findings this branch did not inherit from main, and cargo fmt --check is clean.

Not verified

The sweep paths were exercised through unit tests against temp directories, not a real update on a packaged build. The logic is small and the destructive parts are covered, but nobody has watched an actual version change free the space.