PPR capture: tags recorded after an await in async shell content miss the shell tag snapshot #676

Problem (consumer-observable)

A cacheTag() recorded only AFTER an await inside an async server component that renders into a PPR shell is not collected into the shell entry — revalidateTag() with that tag will not evict the shell. Synchronous recording (the canonical #648 form) works; the loss is silent and async-only.

Mechanism

The shell capture snapshots the derived context's _requestTags synchronously at collection time (shell-capture.ts — the collected = [...derivedCtx._requestTags] at the putShell site). React renders synchronous components before that snapshot, so their tags land; a tag recorded after an await (microtask or later) lands after the snapshot and is dropped. Pre-existing behavior: async cache()/"use cache" reads at capture share the same window (their propagated tags can also post-date the snapshot). The document cache does NOT share the edge — it buffers the full response body before collectRequestTags, so late tags are included there.

Documented as a known edge in packages/rangojs-router/docs/design/ppr-shell-resume.md ("A tag recorded only AFTER an await …", ~line 370), landed with the render-callable cacheTag() work (#648).

Fix direction

Defer the shell's tag snapshot to a render-complete barrier (the capture already has quiesce machinery — handles settled + Flight task-quiet; the tag snapshot can move behind the same gate), mirroring the document cache's buffer-then-collect ordering. The capture's write path already awaits quiesce before putShell, so the move is ordering-only — no new synchronization.

Acceptance

• Unit (shell-capture suite): an async shell component recording a tag after an await → tag present on the stored ShellCacheEntry; an async "use cache" read at capture → its tags present.
• The existing sync-tag tests and the hole/bake invariant tests stay green (a hole still cannot tag — masked loaders never execute regardless of snapshot timing).
• e2e (dev + production, cloudflare-basic): extend ppr-tag-eviction.test.ts with an async-recorded tag eviction case.

Notes

• Severity: low until async render-recorded shell tags become load-bearing — the sync form covers the canonical use, and data-driven eviction via "use cache" tags recorded on HIT reads is unaffected outside capture.
• Discovered during PPR shell tags via render-callable cacheTag() (API settled: no variant keys — the URL is the key) #648 implementation review; an attempted inline fix was deliberately reverted as out-of-scope after an A/B run proved the sync path needs no change.