Fix parse cache ref race during cloned program updates

[Copilot]

A cloned project update can reuse SourceFile pointers after their parse-cache entries were concurrently released by snapshot disposal. Re-refing those reused files with Ref could panic with cache entry not found.

• Parse cache

  • Added RefValue, which refs an existing entry or restores it from the reused value if it was deleted before the ref.
  • Added RefIfPresent for optional duplicate-source bookkeeping refs.

• Program clone path

  • Reused source files now restore parse-cache ownership from the SourceFile pointer.
  • Duplicate source entries use optional refs because they only represent previously acquired cache entries and do not carry a restorable SourceFile.

p.host.builder.parseCache.RefValue(
    NewParseCacheKey(file.ParseOptions(), file.Hash, file.ScriptKind),
    file,
)

• Coverage
  • Added focused refcount cache coverage for restoring a deleted entry without reparsing.