refactor(blocks-engine): add a provider-neutral form materializer seam

[chubes4]

Closes #1554

What

Introduces a typed FormMaterializer contract (markup, unsupported, diagnostics) and threads it through SiteToThemeOptions and SectionRenderOptions, so a consumer can decide how captured forms are materialized. Jetpack Forms becomes an explicit named adapter (jetpackFormMaterializer) rather than logic hardcoded into reconstruction.

Why

reconstruct.ts called formToBlocks() directly, so the generic engine could only emit jetpack/* blocks and there was no seam for any other provider.

Default behavior is unchanged

The default remains the Jetpack adapter. An earlier draft defaulted to a reconstructed core/html form; that was rejected because this stack treats core/html in page content as a conversion-quality bug, #220 explicitly targets working form blocks over preserved-but-dead markup, and the change measurably dropped section coverage from 1 to 0 in the reconstruct suite.

Verification

• pnpm exec tsc --noEmit clean.
• Full package suite: 103 files / 496 tests pass.
• Engine reconstruct DLA baseline parity: cases=35 diffs=0, so emitted markup and provenance flags are byte-identical to trunk.
• Added coverage proving a consumer-supplied materializer is honored and reports unsupported controls.

AI assistance

OpenAI gpt-5.6-sol running in OpenCode designed and implemented the seam, corrected the default-behavior regression, and ran the verification above. A human directed the work and reviewed the result.