chore(planning): archive shipped specs/plans + close roadmap post-0.7

All planned epics (3, 4, 5, 6) closed as of v0.7.0 — move the 7 shipped
specs and 7 shipped plans from planning/{specs,plans}/ to
planning/archive/{specs,plans}/ and rewrite cross-references.

- engineering.md: §1 notes v0.7 + live RTD URL + Epic 3 closed; §6
  replaces stale RecordedTransport bullet with the actual
  httpx2.MockTransport pattern (RecordedTransport was deleted in the
  v0.2 pivot per §5); §8 renamed 'Surviving' -> 'Shipped (all epics
  closed)', Epic 6 marked SHIPPED (6-2 docs site live on RTD; 6-3
  benchmarks + 6-5 release flow RETIRED).
- deferred-work.md: trim closed sections; keep only the single Open
  decoder-side item (dormant unless a configurable PydanticDecoder
  use case appears).
- Release notes 0.4.0 / 0.5.0 / 0.6.0 / 0.7.0: References paths
  rewritten to point at the new archive/ locations.

No source-code changes. mkdocs --strict clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512