test(matrix): stacks + storage-presign block integration tests (W4) (#248)

Closes the remaining P0/P1 API matrix legs: stacks (D2 over-cap 402, D4 anon
NULL-team+6h TTL, D11 multi-service persist+env-tag, D12 env-merge counters /
auth / deleting-409) and storage presign (C16 tenant-prefix-scoped, ≤1h TTL,
cross-team reject, broker no-long-lived-cred, non-storage 400, inactive 410,
unknown 404, path-traversal reject). New test files only; no source edits.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mastermanas805