worker offline start: first activities on reveal-prefetched nodes

[zgeoff]

The reveal-time seed mint (#509) prepares nodes server-side, but nothing yet tracks the client half of the promise: a worker that starts a first activity on a prefetched node while offline and reconciles when the connection returns. Today an activity row only ever comes from a live start response or a resync anchored on an existing row, so the first step into fresh territory still requires a connection.

Player story

You're away from a connection, and you finish clearing the place you're in. You move on into ground you'd already uncovered and start fighting there right away — no pause, no "come back online first". When your connection returns, everything you did while away lands once: the runs you started offline count exactly as if you'd been connected, nothing is doubled, and nothing you earned is lost.

In one sentence: ground you've already uncovered is playable the moment you arrive, connection or not, and it all settles up cleanly when you're back.

Scope

• worker starts a first activity on a revealed node offline, deriving the simulation input locally from the reveal-prefetched genesis seed instead of a live start call
• server rows for offline-started attempts are created at reconnect and checkpoint batches delivered in chain order, deduplicated exactly like resubmitted live batches
• resync and fast-forward can anchor on a scope with no server row yet — today both require an existing row
• offline continuation across node boundaries (finish one node, travel, start the next) within the offline-progress cap; the prefetch radius (seed chain: revealNodes genesis mint at reveal + prefetch; retire genesis-on-first-start #509) bounds how far this reaches
• offline cap accounting unchanged: the meter debits on append, exactly as for live and fast-forward play

Notes

• Depends on seed chain: revealNodes genesis mint at reveal + prefetch; retire genesis-on-first-start #509 (genesis mint at reveal + prefetch) and, through it, rebuild lib-worldmap-core: deterministic hex-lattice generator #191 (worldmap node model).
• Interacts with continuation dead-ends after an unacknowledged terminal append #582 (the continuation boundary handling this builds on) and offline fast-forward ignores the avatar's retry preference #581 (fast-forward failure action).
• Non-goal: offline loot locality — that's self-found offline play: device keys, local rolls, verification #470 (self-found offline play).