fix: stale weight digest reuse in WeightBuilder cache invalidation

[erayack]

Summary

Fix stale weight digest reuse in WeightBuilder cache invalidation.

What changed

• Added SourceMtimeUnixNano to WeightFile (sourceMtimeUnixNano,omitempty).
• Cache lookup now matches on name + size + sourceMtimeUnixNano (not just name + size).
• Lockfile updates now persist sourceMtimeUnixNano with digest/size entries.

Compatibility

• No lockfile version bump.
• Existing lockfiles without the new field still load; first rebuild becomes a cache miss and rewrites entry.
• Older code safely ignores the new JSON field.

Scope

• Only build-cache correctness path (weights build, weights push, OCI-index build path).
• No behavior changes in weights inspect, pusher logic, or OCI annotations.

Tests

• Updated pkg/model/weight_builder_test.go cache tests to cover mtime-aware hit/miss behavior.
• Verified lockfile entries include the new sourceMtimeUnixNano field.
• Updated integration harness lockfile fixture shape in integration-tests/harness/harness.go.