Prototype voluntary seat handover with durable private retries

Let a current owner issue, cancel or replace a private 24-hour one-time offer.
Recipients accept the same faction and saved game while old browser sessions,
recovery keys and entry receipts lose authority. Preserve exact claim retries
across refresh, restart and later offers, with private tab-scoped continuation.

Fence every claim mutation to its winning transaction. Add durable receipts
and additive backfill for already-issued offers without changing game JSON or
existing seat authority. Block competing navigation during unresolved owner
operations; remove invalidated owner kits on confirmed recovery.

Record the pending CHOAM Nexus after-victory secrecy/window decision without
inventing the unresolved gameplay or opening mode/publication gates.

Validation: types/lint and 4,364 offline tests passed; production build passed;
43 HTTP tests passed. Added 19 offline and 3 HTTP regressions. Independent
privacy/persistence review passed, including deterministic CAS interleavings,
rollback, legacy migration and exact receipt restoration. Browser room 2EMYG4CY
retained private setup choices after handover/refresh and continued into Storm;
follow-up recovery removed its invalid offer with cards/forces/spice unchanged.
All 218 opening rooms preserved (291 after QA and both broad runs). The opening
456 seat, 49 recovery-key, 56 recovery-receipt and 106 entry-receipt rows were
also verified unchanged. Server reused; no restart or recurring automation.

Final private report: ~/.local/share/dune/checkpoints/20260913-seat-handover/
verification-final/report.json. Source fingerprint:
8949cdf58eb8bc8bd33f2657c6b21352b3ee5e17e2a556c684a89073015632f4.
Check/build/HTTP duration: 140565/6689/24835 ms. The earlier broad run passed;
a final recovery-UI review fix required the second source-bound run.
Goal aggregate counter 64,301,976 -> 65,031,751 before Git (729,775) for this
bounded capability and its concurrency/recovery review; full goal remains active.

README.md

@@ -28,6 +28,8 @@ New create/join requests save a private, tab-scoped retry record before sending.
 
 An uncertain saved-kit recovery keeps its exact retry details in memory and prevents conflicting navigation. If that recovery has become obsolete, deliberate abandonment releases the controls after explaining that it cannot undo a completed recovery. Keep a valid saved kit before discarding retry details.
 
+Use **Pass your seat to another player** for a voluntary one-time transfer. The private offer lasts 24 hours; acceptance preserves progress and revokes the old owner’s sessions and recovery kits. The recipient uses **Accept a seat handover** on the home page, then creates their own recovery kit. Keep the tab open until acceptance is confirmed; exact private retry details survive refresh in that tab. See [seat handover](docs/SEAT_HANDOVER.md) for cancellation, retry behavior and current limits.
+
 ## Verify changes
 
 Use the [development guide](docs/DEVELOPMENT.md) for architecture, focused tests