feat(bfld): ADR-141 privacy control plane — modes, actions, attestation (#845)

- privacy_mode.rs: PrivacyMode (RawResearch/PrivateHome/EnterpriseAnonymous/
  CareWithConsent/StrictNoIdentity) layered over the existing 4-class
  PrivacyClass; each mode pins target_class + enforced PrivacyAction bitset +
  soul_signature_enabled
- PrivacyAction enum (Allow/SuppressIdentity/ReduceResolution/DropRaw/AggregateOnly)
- PrivacyModeRegistry (std-gated, heap audit log per ESP32 no_std convention):
  active-mode source of truth, is_action_enforced(), set_mode() appends
  hash-chained PrivacyAttestationProof (BLAKE3, ADR-010), verify_chain()
- no_std-safe: PrivacyMode/Action/AttestationProof are heap-free; registry
  std-gated. Builds --no-default-features AND --features std.
- 6 tests incl. tamper-detection; workspace 0 errors

Co-Authored-By: claude-flow <ruv@ruv.net>