Post-campaign simplification pass: remove the scar tissue the seeding/handoff iterations left behind

The wire-faithful seeding campaign iterated through three handoff designs and several instrument changes in quick succession. Once the in-band capture lands and is proven in the sandbox, do one deliberate whole-codebase review + simplification pass rather than letting the residue calcify. Known candidates (grow this list during review):

- `recorded_output`'s `sql` parameter's handoff role is gone (in-band supersedes it) — confirm its remaining uses (state keys, args) and simplify the signature if any role is vestigial.
- `DejaLoadConnection::inner()` / `into_inner()` — unused after the metadata delegation (#48/#49); delete if nothing needs them.
- `WireSlot`'s API surface after in-band: it becomes an internal detail of deja-diesel; shrink visibility to match.
- Certificate vocabulary drift: `skipped` covers several distinct causes (miss-result, kill-switch, unrepresentable value) — consider named reasons like `not_a_precondition` got.
- Dead artifacts of deleted designs: any leftover docs/comments referencing the registry or task-local scope; the design doc's superseded sections should point at the final shape.
- Test suites accumulated per-iteration (probe files, no_eviction, in-band) — consolidate to one coherent suite that documents the final contract, not the journey.
- readback_missing (218 on the last run) and readback_mismatched (105) — quantified but never classified; fold triage into the pass.
- Vendor: `deja_route_replay_schema` round-trips per checkout — after the churn, re-verify the checkout hook is minimal.

Gate: nothing lands until the sandbox verdict run passes with the in-band capture, so the pass simplifies a WORKING system.

First wait for the in-band capture to pass the sandbox verdict run, then review recorded_output, DejaLoadConnection::inner()/into_inner(), WireSlot, the design document, accumulated test suites, readback_missing/readback_mismatched, and the vendor checkout hook. Done means the working system has no vestigial APIs or superseded documentation, a coherent final-contract test suite, classified readback results, and a minimal hook.

Written by the indexing model from the issue text.