docs: post-PR#47 session handoff — W006 complete, design-return due

[erikshafer]

Session-handoff note for picking up after a break, per docs/planning/README.md's conventions (date-prefixed, disposable by design).

Captures the state after PR #47: W006's slice walk is complete and CritterCab has its first cross-service flow. Records the things that live in no other artifact —

• The next session must be a design-or-tidy return. PR Dispatch consumes telemetry.driver-location-updated — W006 slice 5, CritterCab's first cross-service flow #47 was the fourth consecutive implementation PR with no interleave; three candidates ranked, with Driver Profile's workshop as the strongest pull (the ASB half of ADR-018 now blocks on it).
• PR tidy: housekeeping — refresh stale CLAUDE.md status line #43 is now stale in one further respect and will merge cleanly and quietly wrong — the accurate status line is spelled out.
• CI now pre-pulls Testcontainers images, and the list is unenforced: any new fixture must extend it or it silently reintroduces the Docker Hub race.
• Three pre-existing TrackedSession timeouts were widened, and why.
• The proto-compiled-twice arrangement, and the fact that the real coupling is csharp_namespace, not the assembly.
• Methodology: the four-pass review sequence, plus new rules on gate cardinality, proving regression tests fail, and verifying a library's numbers rather than only its API names.

Docs only — no code, no CI, no spec changes.