[chubes4]

Summary

Preserves explicitly associated external <label for> hosts as editable native label blocks instead of lowering them to core/paragraph, and keeps their passive static attributes so authored selectors still match their real subject.

• New AuthoredLabelBlockGenerator (consumer-namespaced companion block) owns external labels with a nonempty for and no nested controls. Static save keeps the <label> host, for, id, classes and inline declarations with no extra wrapper; the editor is standard RichText + useBlockProps.
• Label hosts carry a sourceAttributes map for passive metadata (data-*/aria-* in the canonical name pattern, lang, dir, hidden, role, title, tabindex). Support/SourceAttribute centralizes the selector-identity names and declared-event vocabulary shared by DocumentRootAttributes, SourceDom::eventMetadata, PHP label projection and the generated editor/save filter. Event handlers, data-wp-* directives, URL/executable attributes and attempts to override owned fields are rejected in both PHP and the registered JS save filter.
• SourceAttributeSubjects proves when data predicates resolve to a retained label/document-root host, so the author-selector preparer keeps the original predicates instead of minting captured-state blocks-engine-attribute-* markers.
• External label admission is shared via FormControlClassifier::isExternalAssociatedLabel. Unassociated prose labels keep their paragraph role; wrapping labels and form-owned controls keep their existing contracts.

Fixes #1617.

Root cause

ReadableFormControlBlockConverter emitted external labels as an escaped core/paragraph, so neither the <label> host nor its for association survived. Type, adjacent-sibling, checked-state and pseudo-element selectors lost their subject. Core can't host this natively: core/paragraph always saves <p>, and core/group accepts a label tag but drops for/htmlFor from its attribute schema. A follow-up review also found that the first label host saved only for/id/classes/style, so label[data-role=caption]{…} lost its subject when data-role was discarded.

Before / after

• Before: the parent converter fails the first association assertion (input + label association and adjacency survive compilation). The first label host fails reviewer reproduction preserves the actual data-role subject.
• After: across 2 label orders × 390/768/1440 × 6 states (rest, live data-role mutation, expanded/negated predicates, native label click, body context mutation, opposite-order click), Gutenberg-saved markup and the real WordPress document canvas exactly match the source. The comparison covers attribute names/values, association, checked state, label/input boxes, painted text-range boxes, font metrics, paint, pseudo-element attr() content and outline. At rest the label is red 20px/24px at 390, red 22px/28px at 768 and green 24px/30px at 1440. Clicking each label toggles its intended native checkbox.
• New regressions: tests/unit/associated-label.php (24 assertions), tests/unit/associated-label-static-attributes.php (41 assertions), and packages/blocks-engine/src/wp/__tests__/associated-label-save-validity.test.ts (Core capability gap, registered companion parse/save/reopen validity, hostile save maps, browser/native parity).

UBC evidence

This came from the fresh UBC residual-owner audit, where form/filter labels lost their source subject and checked-state paint. The fixtures reproduce that source structure in a neutral form. Full 21-route UBC acceptance is not claimed by this PR.

Verification (rebased on current trunk)

• composer validate --strict: pass
• composer test (PHP 8.3, 2G): pass
• composer test:packaging: pass
• tools/visual-parity npm test: pass
• Package suite (typecheck, focused label test, test:bundle, test:svg-decode) passed on the candidate before the rebase. CI re-runs it here.

AI disclosure

AI-assisted under Chris Huber's direction. OpenAI GPT-6.1-sol, working through OpenCode, did the diagnosis, implementation and verification of both commits. Anthropic Claude (Opus), also through OpenCode, rebased onto current trunk, re-verified, and wrote this PR. Chris Huber directed the work and authorized landing.