[wieslawsoltes]

Problem and change

Code OSS startup requires a retained User Timing timeline. WebScene returned entry-shaped values but did not retain or query marks and measures.

This change installs a bounded, realm-owned mark/measure timeline before application code can capture Performance methods, shares the native monotonic clock across top-level and iframe realms, and covers ordering, overloads, clears, cloned detail, invalid timestamps, retention, idempotence, and performance.

Closes #72. The equivalent implementation remains in draft consolidation PR #76.

Validation

• One focused implementation commit on current main at 5e1be233; exact head d727fe33
• Cumulative Release V8 native engine build
• RuntimeCompatibility suite passes 25/25 on the cumulative stack
• Focused native performance-timeline filter: 10/10 local passes; warm runs 50-54 ms
• Persisted gate sustains 50,000 marks within the 1,500 ms budget; current Node run 29.9 ms
• git diff --check

Native GitHub stack #159

1. Implement retained Performance Timeline for Code OSS #134 — current layer — Performance Timeline (Implement retained Performance Timeline required by Code OSS startup #72)
2. Implement FileReader over native Blob bytes #135 — FileReader over native Blob bytes (Implement FileReader over native Blob bytes for Code OSS WebSockets #73)
3. Implement mutable stylesheet CSSOM for Monaco #136 — cumulative top — mutable stylesheet CSSOM (Implement mutable stylesheet CSSOM for Monaco and Code OSS #74)

Only the cumulative top PR receives remote validation. Merge all layers atomically through Stack #159 after the top related checks pass.