guest sdk: large-room callbacks — roster-epoch ctx mode + declared heartbeats + Rust parity

[BCook98]

The kit half of the large-room-callbacks change (engine half follows; design lives in the platform's spec workspace). Targets the two measured per-callback costs in large rooms plus heartbeat flexibility.

1. Ctx roster-epoch mode — opt-in via GameMeta.CtxFeatures

The host re-sends the full member list in every callback payload (~100KB/callback at 1000 players — the dominant input-path cost, and the residual -gc=leaking leak). With kit.CtxFeatRosterEpoch declared, the member section becomes sentinel-keyed (ABI.md §4.1):

memberCount	meaning
0x0000..0xFFFD	legacy full roster — byte-identical to today
0xFFFE	full roster at a u32 epoch
0xFFFF	unchanged since epoch — 6 bytes total

Both SDKs decode the sentinels into an epoch-aware roster cache (zero member allocations on the unchanged form); full forms are authoritative; an epoch mismatch logs once and degrades to the cached roster. Legacy guests never see the new forms; pre-feature hosts keep working via the retained byte-skim cache. ABI major stays 2.

2. GameMeta.HeartbeatMS — declare your tick

Validated (0 or 20..1000) at meta encode; rides a new always-written, presence-guarded meta trailer with the features bitset (ABI.md §4.2). shellcade-kit play honors it locally. Host-side precedence: admin host.heartbeat_ms > declaration > 50ms default.

3. Rust crate parity (including a v2.5.0 gap)

v2.5.0's rosterCap 16→1024 + lazy baselines shipped Go-only — the Rust crate still silently dropped send for index ≥ 16. This PR brings the Rust crate to full parity: ROSTER_CAP 1024, per-slot lazy baselines, allocated-only broadcast reconcile, both new meta fields, sentinel decode, and an Rc-shared epoch roster cache. Golden vectors pin Go/Rust meta encodings byte-identical (trailer bytes cross-checked against the actual Go encoder).

Also

• GUIDE.md: new "Large rooms" authoring section — declare heartbeat + feature, render-on-change dirty tracking, Members() lifetime contract.
• Tests: wire sentinel round-trips + legacy byte-identity golden + trailer presence-guards both directions; SDK epoch-cache behavior incl. AllocsPerRun ceiling on the unchanged form; meta validation panics; Rust sentinel decode + trailer golden + lazy-reconcile contract. go test ./... and cargo test green.

Sequencing: merge + tag v2.6.0 → engine PR pins it.

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