ci: close four release-integrity gaps

[thcp]

Fixes #517. Independent; branches off 0.16.1.

1. Deno pinned and checksummed

build/Dockerfile:38 fetched from releases/latest/download/ with no version pin and no checksum, into every published GHCR image. Deno is the JS runtime yt-dlp feeds YouTube's challenge payload to, so it executes against untrusted input.

The project's own standard already covers this -- make-runtime-pack.sh:186-194 pins QuickJS by asset and SHA256 and verifies with shasum -c, with the rationale written out in a comment. Docker was the one path that did not follow it.

Now pinned to v2.9.6 with a per-arch SHA256, verified before unpacking:

x86_64-unknown-linux-gnu   394f07f4da2bebe6ce6f1e7ce0fa16429b29b08c35e3fac3fe25972676dff4b2
aarch64-unknown-linux-gnu  9a46afc6c392c7cd2ff71a31558935545b46408d0e87f7a86908c712721c046e

Both verified against the published artifacts, including a negative control confirming a wrong hash is rejected by the exact sha256sum -c form the Dockerfile uses.

2. Releases can no longer publish an incomplete asset set

action-gh-release defaults fail_on_unmatched_files to false, and neither Linux nor Windows asserted its files existed first -- macOS has 8 test -f checks, those two had none.

A missing updater asset therefore published a release that went green while the in-app updater 404'd for every installed user.

Both workflows now verify every asset before uploading, and set fail_on_unmatched_files: true as a second line.

3. PowerShell failures are no longer ignored

make-portable.ps1:12 set $PSNativeCommandErrorActionPreference, which exists only in PowerShell 7+. CI invokes the script with powershell -- Windows PowerShell 5.1 -- where it does nothing, and $ErrorActionPreference does not cover native commands either.

A failed CPU-torch --force-reinstall was ignored, leaving whatever torch was already resolved in place, so the zip labelled CPU shipped a non-CPU torch. The later import checks still passed, because torch imports fine either way.

Assert-LastExitCode now guards the venv creation and all three pip installs.

Why not just switch to pwsh: that was the tidier fix, but nothing in this repo uses pwsh and the runner is self-hosted, so there is no way for me to confirm PowerShell 7 is installed there. Switching could break the release outright. An exit-code check works on both versions.

4. trivy-action pinned

ci.yml:181,193 used @master -- the only unpinned action in the repo, while everything else is SHA-pinned. Now on v0.36.0's commit ed142fd.

Verification

yaml.safe_load   ci.yml, linux-release.yml, windows-release.yml all parse
deno hashes      both verified, plus a negative control

Two things I could not verify locally, worth noting:

• PowerShell syntax is unchecked. pwsh is not installed on this machine, so make-portable.ps1 was edited without a parser pass. The changes are a function definition and four call sites, but a second pair of eyes on that file would be worth having.
• The Dockerfile was not built. The Docker daemon is not running here. The Deno logic was verified by running the identical curl/sha256sum -c sequence outside a container, but the layer itself has not been exercised.