Skip selector recascade for identical element IDs #551

Parent: #243. CSS epic: #235.

Proven hot path

Unchanged Code OSS list rendering unconditionally writes a stable row ID during several rendered-range/update paths. WebScene's setAttribute('id', sameValue) and reflected element.id = sameValue both entered attribute-selector transition planning even when the ID was exactly unchanged.

Scope

When an existing ID exactly equals the incoming WTF-8 value, preserve feature diagnostics and custom-element attributeChangedCallback, then return before selector-transition recascade scheduling. Preserve mutation for absent ID plus empty string and all changed values. Add no cache or retained state.

Acceptance

• Focused source and regression are authored.
• git diff --check passes.
• 4,096 connected plus 4,096 detached alternating same-ID writes deliver all reactions with zero transition schedule requests, layout passes, or publications.
• Reconnect preserves ID reflection, document lookup, and ID-selector computed width with one bounded layout/publication.
• Unchanged Code OSS runtime A/B/ABBA, browser/package, lifecycle, memory, pixel, and physical-performance gates run in the deferred lane.

No build, test, benchmark, or formatter was executed.