custom-block generation first slice: core/html → PHP-only dynamic block via classifier, fed to companion_plugin_payload

[chubes4]

Refs #497

What

First slice of custom-block generation — the producer link of the classify → route → generate chain (epic spine #497, keystone #491). The classifier (#258 wiring) already runs at the core/html fallback point and attaches a verdict; ArtifactCompiler already emits a companion_plugin_payload that SSI scaffolds into PHP-only dynamic blocks (#240 / #492 / #498 / #500). This PR adds the missing producer link: when the classifier says a core/html-fallback subtree is a custom_block, GENERATE a block definition, emit a block REFERENCE instead of raw core/html, and add the definition to companion_plugin_payload.blocks[].

Generation flow

At the core/html fallback decision (the unsupported_element path — a subtree that mapped to nothing native/Automattic), FallbackEmitter::maybeGenerateCustomBlock() consults the already-wired SubtreeClassifier. When it returns bucket=custom_block with confidence ≥ 0.7 and the sanitized subtree is non-empty:

1. Generate a dynamic (PHP-only) block via the new CustomBlockGenerator: a generic, structurally-derived local name (e.g. collection-<sig8>), a single editable content attribute, supports.html=false, and a server-rendered render.php (wp_kses_post($attributes['content']) inside get_block_wrapper_attributes()). Dynamic/server-rendered → no save() mismatch.
2. Emit a self-closing dynamic block reference in output (blockName = <namespace>/<local>, attrs only, no innerHTML) instead of core/html.
3. Add the definition to companion_plugin_payload.blocks[] — HtmlTransformer surfaces source_reports.generated_blocks; ArtifactCompiler threads them into CompanionPluginPayload (verified against the SSI consumer's name / block_json / render contract) and namespaces references to the per-site companion plugin (ssi-<site_slug>) so emitted refs match the registered blocks.

Gate + dedup

• Conservative gate: generate ONLY when bucket=custom_block, confidence ≥ 0.7 (the classifier's own MIN_SCORE/MARGIN already caps UNKNOWN at 0.4), and nothing native/Automattic matched. Otherwise keep the existing fallback behavior. Generic only — name/title derive from structure, never fixture/site strings.
• Dedup by structural signature: a tag-only DOM skeleton (ignoring text/attributes) keys a per-transform registry. Same-shape subtrees → ONE generated block TYPE, reused; per-instance content rides each reference's attrs. No near-duplicate block types ("no zoo").

Before / after (qualifying subtree)

Input <my-pricing><div class="tier"><h3>Basic</h3><p>$9</p></div> …×3… </my-pricing>:

• Before: dropped to an html_unsupported_element fallback diagnostic (raw-HTML-equivalent), no block, no payload.
• After: one generated type collection-623e0f92 (title "Custom Collection", dynamic render.php) in source_reports.generated_blocks / companion_plugin_payload.blocks[], and a self-closing reference <!-- wp:ssi-<slug>/collection-623e0f92 {"content":"…"} /--> in output. Two identical <my-pricing> blocks → one type, two references.

Scope

Wired only at the unsupported_element fallback for a minimal, green end-to-end slice; the div/section paths already convert repeated content natively (e.g. html-class-grid-card-layout), so they are intentionally untouched. Classifier + Style//Classification/ internals are read-only.

Verification

Intentional behavior change for qualifying subtrees (core/html → generated dynamic block + payload entry). Baseline composer test:canonical + composer parity (131) were green first; no existing fixtures changed.

Added tests:

• html-custom-block-generation — high-confidence custom_block fallback → generated dynamic block reference + generated_blocks entry.
• html-custom-block-generation-dedup — two identical shapes → one type, two refs.
• artifact-custom-block-companion-payload — end-to-end → matching companion_plugin_payload.blocks[] entry (SSI-shaped, namespaced block_json.name).
• html-custom-block-low-confidence-unchanged — negative control: weak signals stay unknown, fallback unchanged.
• tests/unit/custom-block-generator.php — generator shape + gate + dedup.

Full composer test green (135 parity fixtures), php -l clean.

DO NOT MERGE.

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