refactor(hmr): unify the Swift<->JS scalar crossing behind one JSScalar type

[zzal]

Summary

Closes Part I Wave 3 #4 of the arch+DX audit. The "which JS primitive is this" rule was re-derived in three JavaScriptKit-only paths — encodeStateMapToJS, HMRBridge.decodeStateMap, and JSAdapter's PatchPayload.Field marshalling — alongside a parallel PropertyValue taxonomy. The subtle rules (Bool-before-Int, HMRNilSentinel↔null, integral-number→Int) lived where no unit test could reach them — the exact gap the "Blocker 2/3" HMR coercion bugs shipped through.

Approach — unify and close the coverage gap

A closed core JSScalar enum (string/int/double/bool/null), split so the fragile classification becomes host-testable:

• core JSScalar(stateValue:) / .stateValue — the @State-snapshot side, carrying the Bool-before-Int ordering (Bool bridges to NSNumber, so as? Int-first would misclassify it) and HMRNilSentinel↔null;
• PropertyValue.jsScalar — DOM properties map to the same taxonomy;
• a thin, wasm-only JSScalar(jsValue:) / .jsValue crossing in SwiflowDOM (the integral-number→Int rule), keeping Swiflow core free of JavaScriptKit.

The three crossings now route through it — four hand-rolled primitive-mappings collapse to one. _hmrCoerce is left untouched: on inspection it's a different concern (decoded-Int → Double-@State target coercion), not a JS crossing — noted in code.

The win beyond de-dup: the previously-untested, previously-buggy classification is now unit-tested, so a coercion slip is a red test rather than silent HMR-state corruption found by hand.

Test plan

• 6 new host tests pinning the extracted Any↔JSScalar coercion: Bool-before-Int, HMRNilSentinel↔null, unsupported→nil, stateValue round-trip, PropertyValue.jsScalar.
• Behavior-preserving: every existing HMR round-trip test still passes — they simulate decodeStateMap's output, which JSScalar reproduces byte-for-byte.
• Full Swift monorepo suite 1367/274 (+6).
• HelloWorld wasm build + link clean (the crossing compiles; swift package cleaned the example first for the new JSScalar.swift).
• Counter Playwright suite 11/11 — real-browser property marshalling (TextField value / Checkbox) through JSScalar.jsValue.

Note: the wasm JSValue↔JSScalar crossing itself is still build-only-verified (no WASM test target exists) — but it is now thin and centralized, and the fragile Swift-side rules moved into the host-tested core, a net reduction in untested surface.

🤖 Generated with Claude Code

[github-actions]

📦 Bundle size

Artifact	Baseline	This PR	Δ
App.wasm	5.06 MB	5.15 MB	+1.90%
App.wasm (gzip)	1.80 MB	1.84 MB	+2.21%
JS runtime	54.5 KB	54.5 KB	+0.00%
JS runtime (gzip)	11.3 KB	11.3 KB	+0.00%
Total (gzip)	1.81 MB	1.85 MB	+2.19%

✅ Within budget (≤5% growth allowed).

_{Baseline: Swift 6.3, WASM SDK 6.3-RELEASE, measured 2026-06-18.}