Populate companion preserved_js from runtime islands (+ fix silent JS loss in generated blocks)

[chubes4]

What

Populates CompanionPluginPayload.preserved_js[] from runtime-island data so behavior JS embedded in generated custom blocks is carried into the SSI companion plugin (scoped per block), instead of being lost. The consumer already merged (SSI #496); this builds the producer half (#488).

Notable: fixes a silent JS-loss bug

While building the island→block association (which did not exist — generated blocks and runtime islands came from disjoint elements, and sanitizeHtmlString stripped <script> bodies from generated content), I found inline behavior scripts inside generated custom blocks were being silently dropped. The fix mints the association at generation time.

Change (php-transformer)

• FallbackEmitter::maybeGenerateCustomBlock() now scans the generated subtree for runtime <script>s and records each as a script island tagged with the owning block's FQN via a generic owner_block field (shared scriptIslandMetadata() helper).
• RuntimeIslandPackageBuilder carries owner_block through (stays product-neutral).
• CompanionPluginPayload::preservedJs() projects islands → preserved_js, gated by Feature parity: explicit preserve-vs-rebuild decision per interactive region (verbatim JS only on verbatim markup) #224: only disposition=preserve + js_handling=preserve_verbatim, skipping telemetry/droppable. Per entry: content (verbatim JS), handle, src. Islands with a matching owner_block are scoped (block=ssi-<slug>/<name>); preserve-worthy islands with no sound owner go to preserved_js_deferred[] with a reason — never scoped by guess, never silently dropped.

Scope (honest)

• Built end-to-end: scripts living inside a generated custom block → scoped, enqueued only when that block renders.
• Deferred (declared): free-standing islands (standalone scripts, canvas/form) — no sound generated-block owner exists yet (reason: no_generated_block_owner), the #488 follow-up.
• External unmaterialized scripts: skipped (consumer requires content). Materialized external: carried.

Verification

• composer test + composer parity → 144 fixtures green; new contract coverage asserts the populated scoped entry, the deferral path, and that telemetry JS never reaches the payload. Parity unaffected (only adds source-report data; serialized output unchanged).

AI assistance

• AI assistance: Yes
• Tool(s): Claude Code (Claude Opus 4.8, 1M context)
• Used for: Verification, the association design, implementation, and tests under human review.

[chubes4]

Parking as superseded. End-to-end verification showed the companion-plugin / preserved-JS path is the wrong mechanism for interactivity: custom-block generation is structurally inert across all 72 website fixtures (0 generated — the generation gate at the html_unsupported_element tail is unreachable), and generated blocks strip <script>/on* via wp_kses_post anyway. The real interactive behavior is delivered deterministically by native conversion (core/navigation responsive overlay, core/details + core/accordion). Closing; branch retained for reference.