test: restore the pin inventory, checksum and lint gates

- .github/action-pins.jsonl: record the dtolnay/rust-toolchain (default
  branch head 02cb101e, 2026-09-12) and taiki-e/install-action v2.87.15
  pins that c2e03059 moved the workflows to, verified with git ls-remote.
- verify_checksums_fragment_accepts_spaces_and_leading_dashes: GNU
  coreutils 9.x shell-quotes names with spaces in sha256sum -c output.
  Accept exactly the plain or quoted whole-line verdict and require no
  FAILED line.
- scripts/br-stress.sh: BSD wc pads its count; report bare integers.
- clippy -D warnings: redundant closure (65f3dfa2) and a collapsible if
  (3a3ad369).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>