Summary

Rearchitects createGlobalStyle to use shared stylesheet groups, fixing a recurring bug family spanning 7 years and 5+ issues (#5695, #3146, #3008, #3022, #2769).

Root cause: Each mounted instance of a createGlobalStyle component previously allocated its own stylesheet group (componentId + instanceNumber). This coupling between instance lifetime and stylesheet slot lifetime meant any mount/unmount cycle could scatter groups across the stylesheet or orphan them. The bug was "fixed" at least 3 times but kept reappearing.

Fix: All instances now share ONE group, registered once at definition time. An instanceRules cache enables selective removal via clear-and-rebuild.

Ordering algorithm

CSS injection order is now fully deterministic:

1. When styled(), createGlobalStyle(), or keyframes() is called, a group ID is allocated from a global counter
2. Lower group IDs always appear earlier in the stylesheet
3. Render order does not affect CSS order — only definition order matters
4. Global style instances share their definition's group, so remounting always returns to the original position
5. Keyframes defined before a component now correctly appear before that component's rules

Changes

• GlobalStyle.ts — shared group with instanceRules Map, computeRules helper, rebuildGroup (clear + re-insert survivors), rules-equal fast-path to skip CSSOM rebuild when CSS is unchanged, rehydration cache-seeding for static styles
• Keyframes.ts — eager group registration via getGroupForId in constructor (uses GroupIDAllocator directly to keep native builds DOM-free)
• createGlobalStyle.ts — useRef-based instance IDs, RSC per-instance style output from cache instead of shared group
• Bundle size: 12.35KB → 12.4KB (+50 bytes, +0.4%)

Breaking changes

• Keyframes CSS now appears before components that reference them (when keyframes are defined first). Semantically correct but changes observable order.
• SSR rehydration markers changed from id="sc-global-X1" to id="sc-global-X". Apps upgrading with cached SSR output may see one-time style duplication until caches refresh.

Test plan

• 568 tests pass across 53 suites (+30 new tests)
• Multi-instance mount/unmount in every permutation order (2→1→0, 0→1→2, 1→0→2)
• Definition order permutations (A→B→C rendered as C→A→B, B→C→A)
• Rehydration: static and dynamic styles surviving instance unmount + prop change after
• StrictMode double-render with static and dynamic interpolations
• RSC: multi-instance, themed, global+styled in same tree, independent render passes
• RSC coverage for StyleSheetManager, ThemeProvider, withTheme, checkDynamicCreation
• Build passes with bundlewatch size check
• Native build: 0 document. references (DOM isolation verified)
• Performance validated via microbenchmarks (rules-equal fast-path saves ~530ns CSSOM ops at 4-7ns comparison cost)

Fixes #5695. Fixes #3146. Fixes #3008. Fixes #3022.