perf(sync-server): fetch R2 payloads concurrently in pullItems

[h4yfans]

Summary

• Replace the serial per-item R2 for await loop in pullItems with bounded-concurrency windows (R2_CONCURRENCY = 25): map over the already server_cursor-sorted rows in fixed-size windows, await Promise.all each window, concat in order. Cuts first-sync / large-pull latency from O(N) × per-object round-trips to overlapping reads, capped so a Worker never fires hundreds of simultaneous subrequests.
• Behavior preserved: output stays ordered by server_cursor, nulls are filtered, and pullItems' signature/return type, toPullItemResponse, and the D1 batching loop are untouched.
• Implements Plan 007.

Test plan

• pnpm typecheck:sync-server → exit 0
• pnpm test:sync-server → 634 pass (45 files), no flake
• 3 new pullItems tests:
  • order preserved across concurrency windows even when R2 reads resolve out of order
  • unsupported-type rows filtered while surviving rows keep order
  • concurrency bounded — 1 < peak ≤ R2_CONCURRENCY (this assertion is red on the old serial loop, so it pins the new behavior)

Notes

• Docs gate: pushed with MEMRY_DOCS_IMPACT_SKIP=1. This is an internal performance refactor with identical inputs/outputs/ordering — no API, contract, behavior, or documented-surface change, so there is nothing to update under apps/docs/src.

Closes #548

[vercel]

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Project	Deployment	Actions	Updated (UTC)
memry-docs	Building	Preview, Comment	Jun 12, 2026 10:02pm
memrynote-landing	Building	Preview, Comment	Jun 12, 2026 10:02pm

[github-actions]

React Doctor found 3 files changed in this pull request, but none matched the files covered by its enabled checks.

Scope: 3 files changed on perf/pullitems-concurrent-r2 vs. main.

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