engine: assemble the facade cold-start identity derivation and live-session wiring

[FSM1]

Umbrella for completing the facade cold-start sequence — deriving the session identity and key material at login and assembling the live session behind the Engine::start surface.

Context

Engine::start is currently the frozen facade skeleton (#623 scaffolded it; lib.rs / facade.rs note that "the full cold-start sequence … lands with the pipeline slices"). It authenticates and wires background work, but it does not yet derive the session identity/key material or assemble the live-session data paths. #739 (Closes #697) wired the liveness re-PUT loop into start but had to defer every piece that needs derived session key material or the live resolve path — see the dependent issues below.

Scope

The cold-start sequence, end to end:

• Challenge-signature login → Web3Auth key derivation (per the auth pillar; api: build identity auth, refresh rotation, and the ops baseline #624 landed identity auth + refresh rotation).
• Session key material: per-scope read/write keys, per-name Ed25519Signers, writer pseudonym signers — the seeded per-scope derivation with key-regression epochs.
• Live-session wiring: the resolve path, the held-record set, liveness-loop composition, and rehydrate-on-boot.
• Determinism preserved: entropy/time enter via seam traits, never a direct clock/RNG.

Dependencies

Depends on (upstream):

• api: build identity auth, refresh rotation, and the ops baseline #624 — identity auth, refresh rotation, ops baseline (merged).
• engine: scaffold seams, fakes, conformance kits, and the facade #623 — facade scaffold + frozen Engine::start surface (merged).

Blocks

• engine: write-body structure sig epoch mismatch over-rejects read-rotated records at the gate #793 — the epoch-mismatch over-rejection is a cold-start / live-session correctness residual of this slice.
• engine: narrow the spawned-loop secret capture to least-privilege and drop task state on engine drop/logout #795 — the spawned resolve-tick loop whose secret capture must be narrowed is spawned by this slice.
• engine: surface resolve-tick outcomes as steady-state trust-violation events #796 — the resolve-tick outcomes to surface as trust-violation events are produced by this slice's driver.
• engine: skip redundant owner-write-seed recovery and re-hold on steady-state Current polls #800 — the redundant owner-write-seed recovery it removes is on this slice's steady-state poll path.
• web: give the wallet method a started-engine surface #938 — the wallet method cannot obtain a SIWE nonce over a dormant API client; the engine worker's
  ApiClient runs with an empty base URL until this slice's auth/config surface wires the real one.

Notes

• Referenced by engine: wire the liveness re-PUT scheduler loop into the facade cold-start #697 / feat(engine): wire the liveness re-PUT scheduler into facade cold-start #739 as "the deferred facade skeleton."
• Consumed downstream by the web shell (web: rebuild the web shell with login and the vault browser #642, web: ship sharing, bin, settings, and invite views over the facade #643) and desktop login (desktop: wire login, mount lifecycle, tray, and the mounted e2e matrix #650), which drive the facade.

Part of the v2 build (v2-build), tracked in #655.

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Remaining work (after #789 landed #748/#751/#750): the live-session resolve-tick driver

#789 landed the facade reads/command/events (#748), the held-record population logic (#751), and the EOL-renewal pass (#750) — all test-proven but dormant until a production driver populates the held set. This umbrella stays open for that live-session wiring:

• Spawn the production resolve-tick loop (focus-window polling) that calls resolve_and_hold so the held set is populated from live resolves in start().

• API base-URL config: the loop's ApiClient is currently built with an empty base URL (harmless while dormant). Wire the real API base (part of the auth/config surface, still Unimplemented) so EOL republication actually publishes.

• Write-seed plumbing: resolve_and_hold needs the unsealed write_scope_seed in HeldMaterial, but the Adopted resolve outcome does not yet carry it — thread it through the resolve/gate path.

• Surface renewal outcomes: the liveness loop currently drops eol_renew_pass's Vec<EolRenewResult> (marked #[must_use] in feat(engine): wire the facade cold-start pipeline and live resolve-tick driver #789 with an explicit call-site suppression), so a LostRace/PublishError is computed but invisible. When this driver wires the held set live, surface those (Event / dead-letter / retry) so a name can't silently lapse toward EOL.

[FSM1]

Dependency edge (must resolve before this facade cold-start umbrella closes): #763 (enforce cold_seed_checked's root-anchor soundness by type before a shared-scope cold-seed caller lands here) and #754 (aggregate run_sweep per-pass outcome buckets — the production sweep/host caller wiring this umbrella grows is the consumer).