web-ui: queue a mid-turn message by default; steer only on request

[ReganBell]

Pressing Enter while a turn was running always steered — it redirected the live turn, which is destructive and was never explicitly asked for. Mid-turn sends now queue by default, with steering available as an explicit action. The queue is the core's durable one, not the browser's: core already parks a second web turn pending behind the running one, so the client's parallel localStorage queue (which could double-send from two tabs or lose a message when the tab died) is deleted and the core queue is rendered instead. RunStore gains inFlightForThread (every non-terminal run for a thread, oldest first — the tail is the queue) and withdraw (one guarded UPDATE so cancelling a queued turn either beats the worker's claim or reports it started, never both). Steering withdraws the queued run first, then signals, so a message is folded into the live turn or runs as its own — never both. Because the queue is durable, closing the tab no longer drops a message and a second tab sees the same queue.

Deployment notes

Release note: first boot rewrites the runs table to add seq (brief lock); mid-turn send now
queues by default
Schema: ALTER TABLE runs ADD COLUMN IF NOT EXISTS seq BIGSERIAL at boot. A BIGSERIAL
column has a volatile default (nextval), so Postgres does a full table rewrite under ACCESS
EXCLUSIVE lock — on a deployment with a large runs table this is a slow boot migration that
blocks run claiming; release notes should mention possible pause on first boot.
Blue-green: old instances insert runs without touching seq (default fills it); the claim query's
added 'seq ASC' tiebreak is compatible. Rollback safe — old code ignores the column.
Behavior flip: mid-turn Enter in the web UI now queues instead of steering; upstream users'
muscle memory changes (Steer becomes an explicit control). Deliberate fix, but zero opt-in.
Client-side localStorage queue deleted: any message sitting in a browser's local queue at
upgrade time is not migrated (edge case, tab-lifetime data).