Casino game kind, credits host functions, CtxFeatCredits (wire revision 7)

[BCook98]

What

The wagering surface for casino games, as one additive minor (wire revision 6 → 7):

• GameMeta.Kind (game | casino) + MaxPayoutMultiplier — a new presence-guarded trailing meta section. Absent ⇒ game, so every existing artifact keeps its meaning; a casino game must declare a payout ceiling ≥ 1 that covers its largest configurable outcome (wire.ValidateGameKind, enforced at SDK encode time and refused at host load).
• Three host functions — credits_balance, credits_wager, credits_settle: a host-owned account-wide wallet. Wagers escrow atomically (a refused wager means the bet did not happen); settlement is gross (loss 0 · push = stake · win = stake + winnings) and clamped to stake × the declared multiplier; feature rounds (free spins, double-up ladders) ride the triggering stake and settle once. Typed refusals: insufficient, economy-disabled (degrade, never trap), denied, unavailable. Game-kind callers are rejected host-side before any service call.
• CtxFeatCredits (bit 2) — declaration-only bit marking an artifact that wagers.
• Go SDK: Services.Credits (+ kit.ErrInsufficientCredits etc.), kittest wallet double (Credits/CreditsStakes/CreditsDisabled), native dev-runner and memsvc backends (so go run ., check, and play all work today).
• Rust SDK: Meta::kind / max_payout_multiplier, Room::credits_*, CreditsError, CTX_FEAT_CREDITS — wire revision bumped in lockstep, cross-language scalar goldens regenerated, hardcoded meta hexes updated.
• Docs: ABI.md §3 (host functions + status codes) and §4.2 (game-kind section), GUIDE.md casino authoring section.

Testing

• go test ./... green (incl. new wire.ValidateGameKind/round-trip/truncation vectors, gameabi kind-gating + status-code mapping tests, regenerated scalars.txt)
• cargo test green in rust/ and crossverify/ (revision crosscheck passes; byte-identity goldens updated from the Go reference encoder)
• End-relative truncation offsets across all older-form decode tests grown by the new 5-byte section

Notes for review

• The multiplier ceiling is a per-hand clamp; the platform applies its own ceiling and economic caps on top — this repo only carries the contract.
• CtxFeatCredits is intentionally declaration-only: graceful degradation against hosts lacking the imports is impossible (instantiation fails before detection), so hosts at this revision always export the functions and signal economy state via the disabled status code.

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