fix: scope dev defer registrations per HTML render to bound registry growth

[uhyo]

Problem

In dev, deferRegistry is a module-level singleton in the RSC environment. Every HTML render registered fresh defer() entries under random UUIDs and nothing ever evicted them, so memory grew unboundedly over a long dev session (many page loads / HMR re-renders).

Closes #144

Approach

Implements the "track registrations per HTML render" strategy suggested in the issue, extended to a rendered-by tree so that invalidating a top-level render invalidates all of its nested defer() registrations too:

• DeferRegistry#startRender(key, fn): the dev server wraps each top-level HTML render and each HMR RSC re-fetch in an AsyncLocalStorage context keyed by entry path. Every defer() registration made during that render is attributed to a RenderHandle for it.
• Rendered-by tree: when a deferred entry is loaded later (during SSR or a client fetch), its element is rendered inside the owning render's context, so nested defer() calls attach to the same top-level render.
• Invalidation with grace period: when a new render for the same entry path begins, the previous render's whole tree is evicted after a 30s grace period, so in-flight client fetches for the previous page load still resolve. Late registrations into an already-superseded render re-arm eviction so stragglers are dropped too.
• Build unaffected: build-time rendering never calls startRender, so unattributed entries are kept until the process exits, as before.

DeferRegistry is extracted from defer.tsx into deferRegistry.ts (with re-exports preserving existing imports) so the lifecycle is unit-testable without the virtual:funstack/config / #rsc-client modules.

Verification

• Typecheck, lint, and all unit tests pass, including 9 new registry lifecycle tests (eviction, grace period, per-key isolation, nested-tree invalidation, build-mode exemption).
• All dev-server (28) and production (31) e2e tests pass.
• Live check against the fixture-ssr-defer dev server: after re-rendering the page, the previous generation's parent and nested payloads kept serving 200 during the grace period, then both returned 404 after 30s, while the current generation stayed at 200 — confirming AsyncLocalStorage attribution holds through real React flight rendering.

🤖 Generated with Claude Code

https://claude.ai/code/session_012sAWXBbDQBMWmdZpgkssXH

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