v0.6.4

Internal quality release: no user-facing behavior changes.

Added

• Trust-boundary test suite (#151): 70 tests covering the modules that are
  the security/verification story. Every sandbox hardening flag now has a
  named test — the "never weaken hardening flags" rule is executable — and
  the clean-room replay is pinned as the only source of "verified"
  (fresh sandbox per attempt, no agent state, marker + exit code both
  required, no marker leakage across attempts).
• CI: coverage ratchet at 80% (measured floor) on the 3.12 leg and a mypy
  type-check job gating at zero errors (#152). The handful of src touches
  required are annotation-only or provably behavior-identical; grounding
  paths audited.