Stop in-flight forks from making the source name ambiguous

[sjmiller609]

Problem

Forking by source name fails under concurrency. Reproduced with 25 concurrent forks of one standby source: 19 of 50 fork calls returned 409 {"code":"ambiguous","message":"multiple resources match, use full ID"}. The same run with the source addressed by ID succeeded 50/50.

Mechanism

fork clones the source's instance directory (copyForkSourceGuestDirectory → cloneGuestDirectoryForFork) and only afterwards writes the fork's own metadata (forkMeta.Id / forkMeta.Name). In between, the fork's directory holds a verbatim copy of the source's metadata.json, including the source's Id and Name.

findInstanceMetadataByNameOrIDPrefix walks every metadata file and counts name matches, so during that window it sees two instances named <source> and returns ErrAmbiguousName. Every concurrent fork widens the window, so the wider you fan out, the more lookups of the source fail.

Fix

• query.go: ignore metadata whose Id doesn't match the directory it was loaded from. A directory that describes another instance isn't that instance, and this also covers any other path that clones a guest directory.
• snapshot_alias_lock.go: skip metadata.json in the fork clone. The fork's metadata is written from forkMeta moments later, so copying the source's was both the cause of the window and wasted I/O.

+11 non-test lines. No API change, and no behavior change for callers that address instances by ID.

Tests

• New lib/instances/resolve_test.go: a directory holding another instance's metadata must not make that instance's name ambiguous, and the fork's own name still resolves once its metadata is written. Passes.
• go vet and gofmt clean; the resolver / list / admission / delete unit tests in lib/instances pass.
• The fork integration tests (TestForkCloudHypervisorFromRunning*) fail in my environment with "Image should be ready after 60 seconds" — they need the root/KVM test environment. I verified they fail identically on a clean checkout, so this change isn't responsible; CI should be the judge.

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