* Studio: reach the published source asset when a mix build's commit 404s
A llama.cpp "mix" prebuilt records a merge commit that is never pushed to
the fork, so the codeload/archive URLs for that commit 404. The merged
source tree is instead published as a release asset alongside the prebuilt
(llama.cpp-source-commit-<sha>.tar.gz). The installer resolves that asset
URL from the approved-checksums manifest, but when the manifest omits the
top-level repo/release_tag the URL resolves empty, hydration falls through
to the 404-ing commit archive, and the prebuilt install drops to a slow
source build (or fails outright).
Extract exact_source_asset_url() and resolve the asset's host and tag
defensively: the artifact's own repo, then the manifest repo, then the
source repo; and the manifest release tag, then the tag we actually
installed the prebuilt from (the source asset is its sibling on the same
release). Normal installs build the identical URL as before, so this only
adds a working fallback for the degenerate manifest.
Add unit coverage for the resolver, including the empty repo/release_tag
regressions.
* Studio: cover exact_source_asset_url through the real parser chain
Add TestExactSourceAssetUrl.test_resolves_through_real_parser_chain, which runs
parse_approved_release_checksums -> preferred_source_archive -> exact_source_asset_url
so a regression in the parser or source-selection wiring cannot pass while only the
hand-built helper unit tests stay green.
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