Convert ButtonLinkDispatchTrait into a ButtonLinkDispatcher collaborator (#242)

[chubes4]

Slice 5 for #242, workstream 1. Follows #1310, #1312, #1315, #1316.

Second trait eliminated, same shape as #1316.

What changed

ButtonLinkDispatchTrait → ButtonLinkDispatcher behind a 15-operation ButtonLinkDispatchContext. All call sites migrated, trait deleted — no forwarding shims.

POSITIONED_FRAGMENT_LINK_CARRIER_CLASS moves from HtmlTransformer onto the dispatcher. The dispatcher owns the rule that writes that class; the transformer reads it back when inspecting serialized output, so ownership now sits with the writer rather than the reader.

Two internal helpers (nonButtonAnchorWrapperAttributes, isPositionedFragmentLink) became genuinely private — they were only ever called from within the trait, but as trait members they were part of the transformer's method surface.

Behavior preservation

385 fixtures — DIFFERING: 0 — threw: 0

composer test exit 0.

Coverage

tests/unit/button-link-dispatcher.php — 15 assertions, constructs no transformer.

The rule worth having a direct test for is the anchor class-identity split: source classes belong exclusively to the saved link, and only generated geometry classes may ride the paragraph host. Previously observable only by transforming a document and reading serialized output. Now asserted directly, including the edge case where everything mapped was a source class and className is dropped entirely rather than emitted empty.

Also covered:

• a runtime-targeted anchor is preserved and skips pattern recognition entirely (asserted by proving the recognizer is never called, not just by the returned block)
• a runtime-targeted button additionally records a control island
• an icon-only anchor survives on aria-label + safe href, but drops when the href is javascript:
• positioned-fragment-link carrier class applies only when position is absolute/fixed, and never when role="button" makes the anchor a control
• dispatch precedence: a linked logo wins over pattern recognition

Impact

	trunk	after
HtmlTransformer.php	12,650	12,684
single-consumer trait lines	8,079	7,960

HtmlTransformer.php grows 34 lines again — the context factory costs more than the use statement. Consistent with the previous slices and still not the metric.

Cumulative across five slices: convertElement() 441 → 301, 13 collaborator files, 147 isolation assertions, two traits eliminated.

Scope

Five single-consumer traits remain: StyleResolutionTrait (2,822), FormDispatchTrait (1,865), NavigationStyleProjectionTrait (1,319), SvgMaterializationTrait (983), NavigationToggleSuppressionTrait (781), ElementConversionTrait (594).

ElementConversionTrait is the natural next one: smallest remaining, and its dependency on the runtime-island analyzer is already migrated by #1316.

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AI assistance disclosure: implemented and drafted by Claude Sonnet 4.6 running in Claude Code, operated by @chubes4. The AI captured the pre-change corpus baseline, converted the trait, migrated all call sites, verified byte-identical serializedBlocks and fallback counts across 385 fixtures, and wrote the isolation test. Reviewed by a human before opening.