refactor(242): extract runtime-island recognition from HtmlTransformer

[chubes4]

Slice of #242, workstream 1 — the epic names "nav-chrome / runtime-island units" as extraction targets.

What moved

Support/RuntimeIslandTrait now owns the runtime-target and runtime-island cluster: canvas and DOM target recognition, app-shell roots, workspace surfaces, bounded data-attribute targets, and the island and preservation records that follow from them.

24 methods, ~404 lines. HtmlTransformer goes 13,242 → 12,858 lines.

Why this cluster

It was already a contiguous band in the file and cohesive by responsibility. Eight non-runtime methods were interleaved in that range and are deliberately left in place — descendantElements (9 callers) and requiredScriptsForElement (5 callers) are shared utilities, and the rest are single-caller helpers belonging to other clusters. Moving only the runtime-named methods keeps the boundary unambiguous.

The three state accessors (runtimeBehavior, runtimeDom, runtimeSelectors) and the scattered runtime predicates elsewhere in the file also stay put; they are embedded in other clusters and are not this slice.

Pure move, verified

The epic requires behaviour-preserving slices with no logic smuggled in. Rather than assert that, I checked it:

• All 24 method bodies are byte-identical to origin/trunk — compared programmatically, signature through closing brace, not eyeballed.
• Method inventory unchanged: trunk resolved 522 methods on the class; class + trait still resolve exactly 522. No method lost, none added.
• The extraction was scripted rather than hand-edited so the moves could not drift.

One necessary change: ShellLandmarkPolicy gains an explicit import. It lived in the same namespace as HtmlTransformer and needed no use there; in Support\ it does. That is a namespace-resolution consequence of the move, not a behaviour change — and it surfaced as a hard fatal in the contract suite, which is how I caught it.

Verification

Full composer test on the php-transformer package, before and after:

• baseline: 289 parity fixtures passed, all contract suites green
• after: 289 parity fixtures passed, all contract suites green, SUITE EXIT=0

Identical green, zero block-output diff.

Base

Taken on a quiet base. #939, #764 and #659 were the open PRs touching this file and were closed as superseded or unsalvageably stale before starting. #1268 is open against HtmlTransformer but has zero hunks in the extracted band, so it will not conflict.

AI assistance

OpenAI GPT-5.6 Sol via OpenCode identified the cluster from method-density analysis, scripted the extraction to guarantee byte-identical moves, and verified the parity suite before and after. Chris Huber directed the work and remains responsible for the change.