fix(cache): Write files atomically and mutex cache reads

[nikolaik]

Changes

1. Atomic writes (fixes read-while-writing). FileStorage.Create now returns a new CacheWriter interface (io.Writer + Commit() + Close()). StoreInPath.Create writes to a temp file (os.CreateTemp in the same dir) and only os.Renames it into place on Commit(). Since rename is atomic on the same filesystem, readers see either the complete old file or the complete new file — never a partial one. Closing without committing discards the temp file, so failed/partial downloads leave no garbage.

2. Ref-counted keyed mutex (fixes write/write + dedups downloads). Cache gained a map[string]*keyLock guarded by a sync.Mutex. lockKey(filename) serializes work per-artifact and cleans up the entry once no goroutine holds or waits on it. On a cache miss, only one goroutine downloads; others block, then serve the freshly cached file.

3. Rewrote ProxyDownload into: verify RepoHead → serve-from-cache fast path → acquire per-file lock → re-check cache → download-to-cache → serve → fall back to direct proxy if caching is unavailable. RepoHead now runs up front on every path, so cached content is never served without an access check.

Prompt

Do we need a mutex or similar to protect against two go routines writing to the same file and/or reading from a file while writing in ProxyDownload?

[Copilot]

Pull request overview

Adds atomic cache-file publication and per-artifact synchronization to prevent partial reads and duplicate downloads.

Changes:

• Introduces atomic temporary-file writes with explicit commit.
• Adds keyed locking and cache-miss rechecks.
• Updates cache test doubles for the new writer interface.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
services/modules/cache.go	Implements atomic writes and synchronized downloads.
services/modules/cache_test.go	Updates mocks for commit-based cache writes.

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