fix: preserve explicit HTML fragments in native intent

[noeltock]

Problem

Explicit Custom HTML in native intent silently becomes an empty block on WordPress 7.1. Its content attribute is local to the editor; serialization now reads innerContent. Small authored SVG and form fragments therefore disappear even though validation reports success.

Solution

Reuse the converter's existing HTML construction helper for intent core/html. Preserve supplied attributes and dropped-attribute diagnostics, keep text precedence, and reject non-string content. Converter fallback behavior remains unchanged.

Behaviour that changes or must stay true	Read first	Proof
Explicit fragment bytes survive intent serialization	src/intent/index.ts	SVG/form regression through assemble, realize, parse and validate
Native block metadata and diagnostics remain visible	createHtmlFallback and intent constructor	Metadata and dropped-attribute assertions
Converter fallback still warns and refuses strict conversion	src/convert/fallback.ts	Cross-producer fragment checks

Diff

3 files, +60 / -6.

 assembleNode(core/html)
- generic createBlock('core/html', attrs, [])
+ validate consumed string content
+ createHtmlFallback(wp, content, attrs)
 createHtmlFallback
+ optional attrs forwarded to createBlock
  existing innerContent/originalContent payload retained
+ SVG/form cross-producer regression; malformed content rejection

Testing & verification

Reviewed revision: d3f6293e97ea2dd36b5611abb1ad966cbe60eebb · Environment: Node 22 locally; WordPress 7.1 / PHP 8.4 in OrbStack.

• npm test -- dev/test/intent.test.ts dev/test/convert.test.ts dev/test/coverage.test.ts: 95 passed.

• npm run typecheck, npm run build, npm run check:private, git diff --check: passed.

• Actual general CLI apply updated a native page containing both localized fragments, with successful full field readback. A real page editor edit, Save and reopen retained both SVG/form fragments with zero invalid blocks; restoring the edit returned exactly to compiled content. Native theme files also saved byte-for-byte unchanged.

• Converter selection and existing fallback behavior are unchanged; this does not add a conversion rule or change benchmark scores.

• Exact-head CI 38301931599: Node 22.13/24, package-boundary, WordPress proof and CI scope passed.

Not verified: no production apply or release. The large-mutation transport fix in #193 is already merged.

Risk / rollout

Callers that explicitly supply non-string Custom HTML content now get a clear failure instead of silent empty output. Detection: exact payload roundtrip checks and visible live fragments. Rollback: revert these three files; that reintroduces silent intent fragment loss and does not undo WordPress writes.