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

[maverox]

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 (Delete DejaLoadConnection's inner/inner_mut/into_inner: implement GetPgMetadataCache instead #48/fix(diesel): delegate the pg metadata cache instead of handing out the connection #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artifacts of deleted designs: any leftover docs/comments referencing the registry or task-local scope; the design doc's superseded sections should point at the final shape.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.