refactor(filesystem): preserve admitted file facts

Return the validated descriptor and its original metadata from regular-file
admission. Propagate that contract through owned reads, hashes, snapshots,
artifact inventory and provider file workflows instead of replacing initial
facts with another stat. Keep arbitrary output-handle operations separate.

Preserve final descriptor/path identity proofs, borrowed-source verification,
BigInt precision, cancellation provenance and failed-close ownership. Remove
the obsolete source and owned-ZIP baseline stages and update their consumers
and regressions. Tie read completion faults to actual reads instead of the
retired duplicate stat; instrument real handles without fake handle wrappers.

Validation: fresh runtime/catalog build; focused source, lifecycle, filesystem,
CLI/MCP and authored provider-fixture tests; full static checks; docs checks;
test discovery; duplication check. Platform-specific cases remain skipped on
macOS. No installed provider or device behavior was verified.

src/reference/ReferenceSourceReaderFile.ts

@@ -248,7 +248,7 @@ const readStableFileEntry = async (
         parentBefore.message,
         safeSize(expected.size),
       );
-    const handle = await openRegularFile(absolute, {
+    const { handle } = await openRegularFile(absolute, {
       symlinks: "reject",
       signal,
     });