Remove incomplete reload/keyed reconciliation

[samuel-williams-shopify]

Reload's in-place, key-based reconciliation was incomplete and not correctly wired, so rather than ship a partial fix we're removing it and will revisit with a simpler design (likely blue-green "adoption": build the new container reusing unchanged processes, stop the rest).

Why

• Controller#reload was orphaned — nothing operational called it (not signal-wired anywhere; SIGHUP → restart), only the tests.
• The container-level reconcile-remove path was broken (Keyed#stop? called @value.stop, which children don't implement → NoMethodError) and would also have respawned restart: true children.
• Mixing keyed and non-keyed spawns accumulated duplicates on each reload.
• Falcon's Virtual#setup uses the keyed API but never actually exercises reconciliation (its SIGHUP → restart rebuilds a fresh container each time), so nothing relies on the working behavior.

Changes

• Remove Controller#reload.
• Remove Async::Container::Keyed and the container-level reconcile (mark/clear/sweep, stopping obsolete keyed children).
• Generic#reload now simply re-runs the given block; existing keyed children are still reused via spawn(key:), so re-running setup does not duplicate them.
• Retain the primitives Falcon depends on: spawn(key:) registration and container[key] lookup ([] now returns the child directly).
• Remove the Controller#reload tests.

Not broken

Falcon only needs spawn(key:), container[key], and container.reload { ... } (which still yields) — all retained. Full suite (191) green.

Supersedes #72 (which fixed the reconcile-remove bug); closing that in favour of this removal.