BtbN publishes new FFmpeg builds without changing the version number.
The shared-minor comparison therefore reported stale artifacts as current.
Compare advertised artifact URLs during resolution and hash changed URLs
when applying the update. Keep dry-run checks metadata-only and preserve
pins for targets without an update source, including Termux.
Verification: 41 focused tests passed. The regression exercises real
archive downloads, installation, retained target pins, and a second
update that performs no writes. Native ARM64 FFmpeg also passed a real
16 kHz audio encode after installation.
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