release: publish v0.48.1 for issue 660

[odvcencio]

Summary

Publish v0.48.1 as an urgent correctness patch for issue #660.

Scope

• Stop inherited-only field entries from labeling anonymous separator nodes.
• Preserve deferred direct repeated fields, including Scala path separators.
• Add production and candidate-route coverage for Python imports and subscripts.
• Update the changelog and the README release status.

Verification

• Docker root regression and field-reduction tests: pass.
• Docker go test ./grammars/... -count=1: pass.
• Python real-corpus C parity: 25 of 25 samples pass with deep parity enabled.
• The full root package retention gate fails on both v0.48.0 and this branch at TestArenaGCRetentionAfterRelease with 82 MB versus a 60 MB limit. The failure is pre-existing and unrelated to this patch.

This branch starts at the immutable v0.48.0 tag. It excludes the unresolved main-branch performance work.