Batch mutable stylesheet publication

[wieslawsoltes]

Repeated CSSStyleSheet.insertRule/deleteRule calls rebuilt and published the owning <style> source after every mutation. In the unchanged Code OSS theme continuation, that repeated work held a required promise checkpoint for seconds and delayed the remote extension-host initialization send.

This change keeps the live CSSOM wrapper and synchronous observable behavior, stages one final serialized source per stylesheet owner, and materializes it before observable style/layout reads or the checkpoint boundary. A first source larger than the 32 MiB coalescing budget takes an immediate nonqueued path, so pending storage stays bounded. It also preserves staged state across disconnect/reconnect, clears it on direct text replacement/navigation/teardown, and accounts for retained wrappers/source text in runtime memory metrics.

Closes #297.

Validation

• Native stylesheet CSSOM, Monaco topology, cascade-scaling, and 31-navigation lifecycle gates pass on exact base 486e7931; merged head was fe4bba7d. The exact-head Linux runner passed the runtime browser compatibility step, and Node 24.18.1 CSSOM compatibility passes 8/8.
• Product-shaped 20-cycle exact-head gate: p95 3.985 ms, max 5.410 ms, layout +22, scene +1, flat heap/RSS, and sync → microtask → timer ordering. The oversized-source regression also passes low-memory cleanup.
• Native WebScene and Chrome 153 contract: 5/5 subtests each.
• Unchanged packaged Code OSS on b69f931d: exact remote root/provider, 5 resolved children, 5 Explorer model children, 5 rendered rows; resolve 97.582 ms, first child 1,487.051 ms, Ready→Initialized 759.817 ms, init send→native WebSocket entry 374.573 ms. The no-candidate control was 5,215.651 ms Ready→Initialized and 3,309.197 ms send→entry.
• The branch was subsequently rebased over CSS metadata PR Generate shared CSS property identity metadata #313, parser/resource-interception PR Intercept connected resources through Service Workers #310, and native CSS property-identity PR Generate native CSS property identity #316. Shared paths and generated metadata semantics were audited; the final rebase was conflict-free and exact-current focused gates pass.

Scope boundaries

This PR does not claim #299's separate Monaco rendered-text defect or close #252's broader workspace/provider surface. Whole-package RSS also remains above the 750 MiB release budget. A same-package A/B shows that budget failure predates this change and the candidate changes peak/retained RSS by only +0.15%/+0.25%; AppScene#42 owns the per-process memory follow-up.

[wieslawsoltes]

Required review amendment is pushed as bc37dbbd.

• Minimal stylesheet owners without getAttribute now initialize media state safely; this restores the existing runtime-compatibility fixture.
• A first serialized source above the 32 MiB coalescing limit now bypasses pending replacement storage: prior pending work flushes, the source is retained once for CSSOM disconnect/reconnect semantics, and a connected owner publishes immediately. The internal stage callback returns false for this immediate path and true for bounded queued staging.
• Focused native coverage stages a 32 MiB + framing first source against a detached retained sheet and asserts the immediate/nonqueued result, disconnected state, and null style.sheet, followed by low-memory cleanup.

Focused validation:

• Node v24.18.1: node --test tests/RuntimeCompatibility/stylesheet-cssom.test.mjs — 8/8 pass, 55.1 ms.
• Certification native gate: WEBSCENE_NATIVE_ENGINE_TEST_FILTER=stylesheet-mutation-performance webscene_native_engine_tests — pass; 20-cycle p95 4.386 ms, max 6.025 ms, layout delta 23, scene delta 2, heap 34,274,148 → 34,274,148 bytes, peak RSS unchanged at 105,299,968 bytes.