docs(s3): mark H3/H4/H5/V5/V6 done; update audit banners + orchestration

- finding-orchestration.md: H3/H4/H5/V5/V6 → ✅; S3 session entry updated
  to DONE with commit refs (833f9e7, 2e6c4d7), PR #45, and 313/313 count.
- test-coverage-error-handling-audit-2026-05-31.md: banner updated to
  reflect H3/H4/H5 resolved in S3; test count 298 → 313.
- input-validation-audit-2026-05-31.md: banner updated to reflect V5/V6
  resolved in S3; test count 298 → 313.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doc/reviewer/input-validation-audit-2026-05-31.md

@@ -12,9 +12,12 @@ Status legend: 🔴 Critical · 🟡 Important · 🔵 Polish
 
 > **✅ Resolution status (2026-05-31, Session 1):** **V1/V2/V4 ✅** (JSON parse +
 > field checks, XML stoi/stod guarded — all surface as `std::runtime_error` with
-> path), **V3 ✅** (NaN/Inf vertex-coordinate guard in `load_mesh`). **V5** (XML
-> strict-reject) and **V6** (DOF-size check) **open** — scheduled S3 in
-> [`finding-orchestration.md`](finding-orchestration.md). 298/298 tests green.
+> path), **V3 ✅** (NaN/Inf vertex-coordinate guard in `load_mesh`).
+> **V5/V6 ✅** (2026-06-01, S3): `load_result_xml` now enforces a strict
+> internal-only XML subset (three format checks; non-conforming files throw
+> `runtime_error` instead of silently returning zeros); `check_dof_vector_size`
+> helper added for the DOF-count mismatch check at call sites.  6 new tests.
+> See [`finding-orchestration.md`](finding-orchestration.md). 313/313 tests green.
 
 > **Threat model:** this is a scientific library, not a network service, so the bar
 > is "fail cleanly and diagnosably", not "resist attackers". But meshes and result