(5) class objects: GC layout descriptor + leak/stress verification

[proggeramlug]

Part of #1785. Design: #1772. Depends on #1786. Coordinate with #1090 / #1096 (@andrewtdiz).

(5) GC layout descriptor + leak/stress verification for class objects

Scope

Register a #1096 layout descriptor / pointer mask for the class object so GC scans it precisely: pointer-bearing slots = static-field slots + the static-prototype/parent link; scalar = class_id. Verify the class object moves correctly under evacuation and the #1095 copying nursery, and is collected when unreachable (the whole point vs. the integer-hack's permanent global-table leak).

Acceptance criteria

• Creating N class objects in a loop and dropping them shows flat/bounded RSS (no per-evaluation leak) under GC stress.
• PERRY_GC_VERIFY_EVACUATION=1 / forced-evacuate passes with class objects live.
• Mixed pointer/scalar layout correctness test for the class object.
• Coordinated with the GC overhaul owner so the descriptor + move semantics match the in-flight GC roadmap: make minor GC structurally cheap #1090 work.

Blocks

(6).