SSR: Phase 0.4 request-safe collection

[dbanksdesign]

Summary

• Refactor sheet state into a pluggable store (sheet-context.ts) so SSR collection can run in isolated scopes
• On Node, typestyles/server and typestyles/build enable AsyncLocalStorage isolation (sheet-node.ts) — concurrent collectStyles / collectStylesFromModules calls no longer share ssrBuffer or dedupe state
• Add async render support: collectStyles(async () => …) returns Promise<{ html, css }> (sync callers unchanged)
• Browser bundle unchanged — sheet-node is only imported from server/build entries; node:async_hooks marked external in tsup

Test plan

• pnpm --filter typestyles test (includes concurrent async isolation test)
• Full monorepo pnpm test
• pnpm --filter typestyles build
• Verify @typestyles/next collectStylesFromComponent still works in an App Router app

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