test: add coverage for payload normalization edge cases

[markm39]

Goal

Add focused tests for normalizePagePayloadForNativeLoad.

Context

Before loading page data into the native engine, the package normalizes blank, malformed, and legacy payloads. This is pure TypeScript logic and is a good place to add confidence without touching native code.

Suggested Scope

• Add a test file under src/__tests__/ for src/payload.ts.
• Cover blank, whitespace-only, invalid JSON, array JSON, missing pages, invalid pages, and valid payload cases.
• Assert both isValid and reasonCode where useful.

Done When

• The new tests fail if invalid payloads are accidentally treated as safe native loads.
• npm test passes.