v5.3.0 — Independence + 3× faster self-compile

Consolidation release: 19 PRs since v5.2.0 (#22–#40).

Headlines

• Independence checker (Phase 3a, #37) — a reference checker written outside Rail re-derives the integer core: the committed seed no longer asks you to trust Rail to check Rail.
• Self-compile 61 s → 19.2 s (3.2×) — parser cursor O(N²)→O(N) (#29) + exact-size GC free-list buckets (#36); autograd/JIT tapes go O(n²)→O(n) (#27/#28).
• Multi-argument lambdas \a b -> e (#33).

Verification surface

• One-command check-me kit: VERIFY.md + tools/verify/check.sh (#32)
• Public reproducible-build check: verify_reproducible.sh (#23), verified cross-host (#22)
• Self-generated truth matrix docs/STATUS.md (#31), grounding contracts (#34), integer div/mod differential fuzzing (#35), leak-guard pre-push (#25)

Docs & robustness: truth reconciliation (#26/#30/#38), latent O(N³) removed from free-var checks (#39), audit walkers de-noised (#40).

178/178 tests. The committed seed remains bit-reproducible — the nightly reproducibility witness stayed green throughout the series. Full notes: CHANGELOG.md.