fix(sdk): make pending-approval consumption atomic

[ra-co88]

What

Pending-approval consumption is atomic: the blob that marks a pending approval is removed with a compare-and-delete whose single winner is the consumer. BlobStore gains compareAndDelete(namespace, key) — true iff this caller's delete removed an existing record.

Why

Approvals are single-use decisions. A read-then-delete pair has a window where two concurrent requests both read the pending approval and both proceed — double execution on one human decision. The compare-and-delete contract guarantees exactly one concurrent caller observes true; the loser sees absent-after-commit.

What changed

• BlobStore.compareAndDelete contract (fail-closed semantics documented on the interface).
• FumaDB implementation: get+delete inside fuma.transaction (real BEGIN/COMMIT on libSQL/Postgres — the serialization is the guarantee).
• In-memory implementation: single synchronous Map op.
• R2 implementation (Cloudflare host): R2 offers no conditional delete, so this is isolate-scoped single-winner (a claim-set gate closing the intra-isolate await window, plus head-then-delete). Residual: cross-isolate races are last-writer-wins; tolerated because the caller pattern is single-consume-per-approval with idempotent re-consume returning null.
• The approval consume path gates on compareAndDelete.

Test plan

Atomicity suite: concurrent consumers — exactly one wins; loser observes absent; no double-execution path. 4 tests green against current main.