Fix reload reconciliation of keyed children

[samuel-williams-shopify]

Reloading a container is supposed to reconcile keyed children against the current configuration: reuse unchanged keys, spawn newly-appeared keys, and stop keys that have disappeared. The stop half was broken.

Bugs fixed

1. NoMethodError on removal. Keyed#stop? called @value.stop, but Forked::Child/Threaded::Child have no stop method (only interrupt!/terminate!/kill!). So dropping a keyed child on reload always raised. Never covered by a test.

2. Respawn of removed restart: true children. Even once stopped, a restart: true keyed child's supervising fiber would immediately respawn it (the restart && !@stopping gate is container-wide). Falcon's virtual hosting spawns exactly these (spawn(restart: true, key: path)).

3. wait_until_ready hang after a reuse/remove-only reload. wait_until_ready slept before checking readiness, so a reload that spawned no new child (nothing sends a readiness message) blocked forever in select even though everything was already ready.

Changes

• Group#stop_child(channel, graceful) — stop a single child with the same multi-phase sequence as Group#stop (SIGINT + wait, then SIGKILL + wait).
• Channel#stopping! / #stopping? — a per-child flag, checked by the supervising fiber's restart gate, so a deliberately-stopped child is not respawned. Lives on the child (whose lifetime matches the flag and which resets naturally on restart), keeping the child-reported state hash free of container control flags.
• Generic#stop_child marks the child stopping and delegates to the group; the supervising fiber removes the entry from @keyed/@state when the child exits.
• Generic#reload snapshots keyed values and stops any that were not re-marked (outside the @keyed iteration, to avoid re-entrant mutation).
• Generic#wait_until_ready checks readiness before sleeping.
• Removed the broken Keyed#stop?.

Tests

Adds two behavioural tests under with "#reload": spawns a newly configured keyed child and stops a keyed child that is no longer configured. The removal test is the regression guard. Full suite (193) green.

Scope note: this implements synchronous (blocking) reap in reload, correct for the current (non-reactor) controller. Once the controller runs under Async, Group#stop_child and Channel#stopping! can be replaced by per-child Async::Task cancellation.

[samuel-williams-shopify]

Superseded by #73 — rather than fix the incomplete reconcile-remove, we're removing reload/keyed reconciliation and will revisit with a simpler (adoption-based) design.