Module migration 

Dual-stack migration protocol.

Migration steps:

1. Allocate new representation alongside old.
2. Initialize new from a snapshot of old.
3. Bump generation, swap strategy tag - both representations now live.
4. Wait for old generation’s in-flight count to drain to zero.
5. Drop old representation.

The MigrationGuard enforces steps 3-5 in scope.

Structs

Generation

Wraps the current generation captured at op entry.

MigrationGuard

Guard for the migration coordinator. Wraps the migrate-then-drain sequence: begin bumps the generation and swaps the tag, wait_quiescent drains the old generation. The drain is explicit - the guard does NOT drain on drop.