fix(precompute): make activation generation-safe

[zzylol]

Closes #596.

Makes runtime activation generation-safe:

• an in-flight window remains owned by its current immutable config;
• replacement configs are staged and promoted only after Tick/Drain closes the old generation;
• old state is therefore serialized with old family/grouping/encoding semantics rather than being relabeled;
• promotion resets delta bases so the new generation begins with a full checkpoint;
• config slice/map/monitor fields are deep-copied before atomic publication.

Updates transition tests to prove scope and same-scope changes drain the old generation and promote at the boundary. Race validation included.

Stacked on #599.

[zzylol]

Follow-up review fix: config publication and window ownership now share a generation RW barrier. Observe holds a shared generation lease; Tick/Drain/activation hold the exclusive lease, preventing an old-config observation from entering a newly activated window.