Refactor form control semantics

[chubes4]

Summary

• Adds a shared PHP form control classifier for control tags, input type inference, and data-entry detection.
• Routes HtmlTransformer form-control helpers and SubtreeClassifier form-control signals through the shared primitive.
• Adds a fixture proving runtime-targeted controls nested in label shells are preserved as runtime islands instead of flattened to static readable text.

Validation

• composer test from php-transformer/ passed, including canonical contracts, unit tests, 230 parity fixtures, and packaging install proof.
• Direct FSE fixture matrix with valid Homeboy DOM capture completed using figma-transformer/scripts/figma-fixture-matrix.php --capture-dom-boxes and php-transformer/tools/visual-parity/bin/dom-box-provider.mjs.

FSE Residual Risk

• This is a maintainability/coverage change, not a fixture-specific risk reduction.
• FSE residuals remain visible: form validity risk 19, SVG/Form/Input islands 98/4/11, missing semantic roles 4.
• DOM capture was valid and CSS loaded; broader visual readiness remains critical due existing layout/responsive risks.

AI assistance

• AI assistance: Yes
• Tool(s): GPT-5.5 via OpenCode
• Used for: Inspected semantic/form/input handling, implemented the shared classifier and fixture coverage, ran validation, and drafted this PR description.