#282 GC: ratio widetag missing from trace/heal/stale/verify-cards dispatches — 'unknown widetag during trace' on any marked ratio

Found while implementing ADR-0028 closure records. gc-object-size-from-header / gc-object-pointer-slot-count / gc-object-movable-p all know +widetag-ratio+ (0x0C), but the four slot-walk dispatches did NOT:

• gc-mark-object (the tracer) — cond falls to (t (error "gc-collect: unknown widetag during trace")), so any ratio reachable at mark time raises.
• gc-heal-*-object — ratio slots never healed (mode-1 minor collections).
• gc-stale-check-object — never scanned.
• gc-verify-cards-object — never verified.

Ratios allocate via gc-ensure/commit-constant-allocation +gc-kind-other+ (emit-ir-make-ratio), i.e. they live in the collectable heap. Likely unseen because ratios are rare and short-lived to date.

FIXED alongside ADR-0028 (same dispatch sites, vector-shaped tagged-slot runs) — this issue records it for the changelog; close when the commit lands.