test: cover normalizePagePayloadForNativeLoad edge cases

[VedantMadane]

Summary

Adds unit coverage for normalizePagePayloadForNativeLoad (src/payload.ts), as requested in #12.

What

New file src/__tests__/payloadNormalization.test.ts covering:

• blank inputs (null, undefined, empty, whitespace) -> valid blank load with blank_payload
• invalid JSON -> isValid: false, json_parse_failed
• array / primitive / null JSON roots -> isValid: false, payload_not_object
• missing or non-object pages -> valid blank load with missing_pages
• valid { pages: {...} } -> preserves original payload
• safety invariant: invalid paths never return a non-blank normalized payload

Also notes the change under [Unreleased] in CHANGELOG.md.

Test plan

• npm test -- 82 passed (including 20 new payload tests)
• npm test -- --testPathPattern=payloadNormalization

Related

Fixes #12