engine: enforce optional cpu trait compatibility

[ironashram]

Adds require_cpu_compatibility, disabled by default. When enabled, destinations must expose every source host HW_CPU_* Placement trait.

• Applies to all four planning paths.
• Rejects missing trait data and caches verdicts per cycle.
• Records CPU traits in snapshots and supports offline replay.

Validation:

• 382 tests pass, plus Ruff, mypy and pyright.
• Removing the guard fails all four planning regression cases.
• Live kol1k test: an incompatible migration fails with a libvirt CPU mismatch when disabled and is blocked when enabled.
• Compatible reverse migration succeeds.
• Snapshot replay reproduces both planning outcomes.

Uses host traits rather than per-instance CPU models. Offline replay requires Placement claims enforcement disabled.