Support primitive page data updates

[spencerc99]

Summary

• Support direct replacement and functional updates for primitive page-data roots.
• Read primitive functional updates from the live page-data store after remote Yjs replacements.
• Preserve object and array draft mutators, including terse returns such as d => d.count++.
• Document the contract and add working visit-counter examples to both starters.

Root cause

Primitive values were held in the page-data map, but the existing replacement helper only mutated object and array proxies. Functional returns were ignored. The initial primitive implementation also retained a stale scalar in proxyByTagAndId after a peer replacement.

Regression coverage

A two-document Yjs test seeds B with cached 0, applies A's remote update to 1, verifies B reads 1, then verifies B's value => value + 1 writes 2.

PR #264 integration

#264 overlaps PageDataChannel.setData for server-gated writes. Keep this branch's root-replacement and PageDataSetter behavior, and derive its gated before snapshot from the same live primitive store value. Then mutate object drafts or use primitive updater returns before buildGatedWriteOps. The server already sends that value as a replace operation. I applied and type-checked that combined hunk against #264.

Validation

• bun run build and bun run test in packages/playhtml: 364 tests passed
• bun run build and bun run test in packages/react: 44 tests passed
• Temporary Identity, authentication & permissions system #264 integration: packages/playhtml build and page-data suite passed

[pkg-pr-new]

Open in StackBlitz

bun add https://pkg.pr.new/@playhtml/common@303

bun add https://pkg.pr.new/playhtml@303

bun add https://pkg.pr.new/@playhtml/react@303

commit: f56fc3f

[cloudflare-workers-and-pages]

Deploying playhtml with    Cloudflare Pages

Latest commit:	f56fc3f
Status:	✅  Deploy successful!
Preview URL:	https://72767ad4.playhtml.pages.dev
Branch Preview URL:	https://cx-page-data-primitive-roots.playhtml.pages.dev

View logs

[spencerc99]

Code review

• P2 packages/common/src/index.ts:399 makes nullable object channels lose contextual typing for functional updates. Because the conditional type distributes over unions, PageDataSetter<Item | null> becomes a union containing incompatible callback signatures for Item and null. In strict TypeScript, a realistic update such as channel.setData(value => value ?? { id: "first" }) now reports TS7006 because value is implicitly any; the previous setter signature contextually typed that parameter as the full Item | null union. Define the callback once over the full T (for example, a single (value: T) => T | void shape or overloads that preserve contextual typing) and add a declaration test for a nullable object channel.