Add opt-in game message delivery confirmations

[justinfernald]

Closes #295

What changed

• add an optional delivery-confirmation argument to controller, host, broadcast, and smart-player game-message APIs
• preserve existing fire-and-forget calls and raw payloads by default
• resolve confirmed sends only after the receiving game iframe acknowledges them
• reject with typed timeout, unavailable-recipient, or communicator-destroyed errors
• wait for every connected recipient during confirmed broadcasts and authenticate host-side acknowledgements against the target player
• document the compatibility requirement and reserved protocol field
• use the repository's established Promise.withResolvers test shim so the new communicator tests run on the Node 20 CI target

Verification

• npx -y node@20.10.0 ./node_modules/vitest/vitest.mjs run --coverage — passed (32 tests) in the exact hosted Node version
• corepack yarn vitest run test/src/message-delivery.test.ts test/src/player-models.test.ts — passed (14 tests)
• corepack yarn vitest run — passed (32 tests)
• corepack yarn build — passed, including declaration generation
• corepack yarn lint:check — passed
• changed-file Prettier check — passed
• git diff --check — passed
• targeted source-and-test TypeScript project — passed
• corepack yarn type-check — repository baseline failure: TS6305 for vite.config.d.ts because the root config includes vite.config.ts while referencing tsconfig.node.json; unrelated to this patch

Demo

Non-visual library change; screenshots and screencasts are not applicable. Reproduce with corepack yarn vitest run test/src/message-delivery.test.ts, which exercises controller-to-host, host-to-controller, multi-recipient broadcast, timeout, disconnect, spoofed acknowledgement, and cleanup behavior.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Remove settled delivery disposers from the player

Each confirmed targeted send calls player.addConnectionListener, which appends its MobX reaction disposer to SmartPlayerModel.destructors. The delivery cleanup invokes that disposer but never removes it from the array, so repeated confirmed sends or broadcasts permanently retain one stopped reaction function per recipient per message. Long-running games using confirmations regularly will therefore accumulate unbounded per-player memory; use a listener path that does not register in destructors, or remove the disposer from that collection when delivery settles.

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