Wire SubtreeClassifier into core/html fallback as measurement (#497)

[chubes4]

What

Classifier measurement wiring on the core/html fallback path — the first wiring step from epic-spine #497. The standalone SubtreeClassifier (#254) is now run on subtrees that fall back to raw core/html, and its verdict (bucket + confidence + top signals) is attached to the existing fallback diagnostic.

This surfaces, across the whole corpus, which core/html dumps the classifier believes should have been native blocks (custom_block / custom_application / etc.) — the data the pattern-recognition swarm optimizes against.

Measurement only — routing/generation deferred

No routing or output changes. Block output is byte-identical; the classifier only adds a classification key to the fallback diagnostic. It runs solely on the core/html fallback emission path (script, canvas, template, inline SVG, form, iframe, unsupported-element), guarded so it never runs per converted element.

Where it's wired

• Diagnostics/FallbackEmitter.php: new public classifyFallbackSubtree() (+ ClassificationContext builder from inline CSS / inline <script> + on* handler JS, and a top-signals projector); attached in captureInlineSvgFallback, captureCanvasFallback, captureScriptFallback, captureTemplateFallback.
• HtmlTransformer.php: attached at the form, unsupported-element, and iframe core/html fallback emission points via $this->fallbackEmitter->classifyFallbackSubtree().
• Classification/ is read-only (untouched).

Example (canvas core/html fallback)

{
  "diagnostic_code": "html_canvas_runtime_fallback",
  "classification": {
    "bucket": "custom_application",
    "confidence": 0.88,
    "signals": { "flags": ["canvas"], "scores": { "custom_application": 3, ... } }
  }
}

Verification

• composer parity: 128 → 128, block output byte-identical (parity asserts expected_blocks for every fixture).
• Full composer test green (contract + 23 classifier unit tests + parity + packaging).
• php -l clean.
• Added a contract assertion that the canvas core/html fallback now carries the classifier verdict and that block output is unchanged.

Refs #497