Collapse nested runtime island findings

[chubes4]

Summary

• collapse nested runtime findings under their preserved owning DOM island
• compare real document identity and node ancestry instead of HTML substring similarity
• retain matching controls that live outside the owning subtree

Verification

• cd php-transformer && php tests/contract/run.php
• cd php-transformer && composer test:canonical

Closes #701.

AI assistance

GPT-5.6 Sol via OpenCode traced runtime selector provenance, implemented the DOM-ancestry deduplication, added regression coverage, and ran the contract and canonical suites. Chris Huber remains responsible for every line.

[chubes4]

Real fixture verification confirms the nested-island collapse.

Homeboy run: 91572cd2-b045-47a9-b595-b40e331f23a9
Reproduction: homeboy runs show 91572cd2-b045-47a9-b595-b40e331f23a9
Combined proof transformer: 476a250517331871e4b5803eb401de082a44b385 (#706 plus fixture capability PR #707)

Fixture 37 results across 7 source pages:

• Runtime islands: 29 -> 15.
• Removed exactly 7 transient .on controls and 7 nested header button controls.
• Remaining stable owners: .site-nav x7, .lang-toggle x7, .filter-chips x1.
• Total findings: 44 -> 30.
• Jetpack readiness and all 9 native form conversions still pass.

The overall fixture gate remains red because SSI currently treats the 15 intentionally preserved stable owners as unacceptable; that acceptance-policy issue is separate from this transformer correction.