GC: test Promise against its production slab slot

Follow-up to #1026.

Problem

Current main 8794ccc5 fails the unit test gc.test.Promise fits the 256-byte GC slab:

expected 256, found 248

Promise is correctly packed to a 216-byte payload. With zig-gc's 32-byte header, Heap.cellAllocationBytes(Promise) is the exact 248-byte request. Production GcCellBacking rounds that request into its 256-byte size class, so the runtime optimization is present; the test incorrectly equates the unrounded request with the selected slab slot.

Exact unmodified-main reproduction:

zig test --dep regex --dep gc -Mroot=src/gc.zig \
  -Mregex=../zig-regex/src/root.zig -Mgc=../zig-gc/src/root.zig -lc \
  --test-filter 'Promise fits the 256-byte GC slab'

Scope

• Expose the production GC backing's selected slab-slot size for a requested cell size.
• Assert that Promise's exact request stays within budget and selects the 256-byte production slot.
• Keep the payload packed; do not add padding merely to make the raw request equal the bucket size.
• Run the focused GC invariant and backing size-class tests.

No-workaround rule

Do not weaken the 256-byte production size-class invariant or add wasted payload padding. Test the allocator decision the invariant actually describes.

Resolved on main by 549c58c0.

The production allocator was already selecting the 256-byte slab: Promise has a 216-byte payload, zig-gc requests 248 bytes including its header, and GcCellBacking rounds that request to its 256-byte slot. The failed test compared the unrounded request with the rounded slot.

The fix exposes the production slab-slot selection, keeps the compact payload, and asserts both the raw budget and exact production size class.

Evidence:

• exact unmodified-main reproduction: expected 256, found 248
• focused Promise fits the 256-byte GC slab: 1/1 passed
• GC cell backing bucket stats attribute size-class pressure: 1/1 passed
• no runtime behavior change