fix(core): displace a container a migration re-key evicts from its slot

[vieiralucas]

Audit fix (cs-crdt-audit MEDIUM, displacement-signal class).

put_slot_lww — the LWW slot installer used by snapshot-migration re-keying — overwrote an occupied slot without displacing the live composite it evicted, unlike Map::set.

Impact: a migration renaming a leaf field onto a key holding a live container (Map/List/Text) with a higher stamp dropped the container from the slot but left it is_displaced() == false. A later op targeting the detached container passed the displaced guard and silently mutated it — diverging from the op seam (whose Map::set displaces the loser) until a snapshot decode ran mark_displaced and healed it.

• Displace the evicted live composite before the overwriting insert, mirroring Map::evict; skip a same-handle re-install (the resurrect loop re-landing a container on its own key). Fixed in put_slot_lww so all three callers are covered.
• Regression tests in state_project.rs: a leaf renamed onto a live map/list/text asserts the evicted container reads DISPLACED; a converge test asserts snapshot-seam and op-seam project identical bytes and that an op targeting the detached container is dropped on both seams.

Fails before the fix (container reads installed), passes after.

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Summary by cubic

Fixes migration re-key behavior so put_slot_lww displaces any container it evicts, matching Map::set and preventing silent mutations of detached containers. Snapshot and op seams now converge when a leaf is renamed onto a key holding a live Map/List/Text.

• Bug Fixes
  • In Map::put_slot_lww, displace the evicted live composite before the overwrite; skip displacement on same-handle re-installs.
  • Added regression tests that a leaf renamed onto a live map/list/text marks the container as displaced and that snapshot-seam and op-seam project identical bytes; ops targeting the detached container are dropped on both seams.

^{Written for commit 545c5e0. Summary will update on new commits.}