feat(adr-118/p1): scaffold wifi-densepose-bfld crate + frame header (3/3 tests GREEN)

Land P1 of the BFLD rollout — the wire-format primitives:

- New workspace member: v2/crates/wifi-densepose-bfld
- PrivacyClass enum (Raw/Derived/Anonymous/Restricted) with allows_network()
  and allows_matter() const helpers reflecting ADR-120 §2.2 and ADR-122 §2.4
- BfldFrameHeader (#[repr(C, packed)]) per ADR-119 §2.1
- BFLD_MAGIC = 0xBF1D_0001, BFLD_VERSION = 1
- BfldError variants for InvalidMagic / UnsupportedVersion / Crc / PrivacyViolation
- soul-signature cargo feature (gated, default OFF) per ADR-118 §1.4
- Compile-time size assertion via static_assertions::const_assert_eq!
- 3 acceptance tests in tests/frame_header_size.rs (all pass)

Bug fix:
- ADR-119 AC1 claimed BfldFrameHeader is 40 bytes. Actual packed layout sums
  to 86 bytes. Updated AC1 and §2.1 prose to match. const_assert in frame.rs
  pins the value structurally — a future field addition that breaks the size
  fails to compile.

Out of scope for this iter (deferred to later P1 commits):
- Field-level missing-docs warnings (21) — addressed alongside accessor helpers
- Payload section parsing — needs the section-length prefix tests
- Round-trip serialize/parse — covered by a fixture-based test in the next iter

cargo test -p wifi-densepose-bfld --no-default-features → 3 passed, 0 failed

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

docs/adr/ADR-119-bfld-frame-format-and-wire-protocol.md

@@ -57,7 +57,7 @@ pub struct BfldFrameHeader {
 }
 ```
 
-Total header size: 40 bytes (validated by `static_assertions::const_assert_eq!`).
+Total header size: **86 bytes packed** (validated by `static_assertions::const_assert_eq!` in `wifi-densepose-bfld/src/frame.rs`). Earlier drafts stated 40 bytes — that was a counting error caught during P1 scaffold; see AC1 below.
 
 ### 2.2 Payload structure
 
@@ -144,7 +144,7 @@ Rejected: CRC must be computed after the payload, so its value would otherwise f
 
 ## 5. Acceptance Criteria
 
-- [ ] **AC1**: `BfldFrameHeader` size is exactly 40 bytes on x86_64, aarch64, and xtensa-esp32s3.
+- [ ] **AC1**: `BfldFrameHeader` size is exactly **86 bytes** (packed) on x86_64, aarch64, and xtensa-esp32s3. The size was initially documented as 40 bytes during ADR drafting — that was a counting error; the implementation in `wifi-densepose-bfld/src/frame.rs` enforces the correct value via `const_assert_eq!`.
 - [ ] **AC2**: 1,000 serializations of a fixed `BfiCapture` fixture produce a bit-identical BLAKE3 hash.
 - [ ] **AC3**: `privacy_class = 0` frame returned through `NetworkSink::publish()` returns `Err(BfldError::PrivacyViolation)`.
 - [ ] **AC4**: Payload CRC32 mismatch causes `BfldFrame::parse()` to return `Err(BfldError::Crc)` without exposing partial payload state.