Implements M1b.5: derive sidecar pid and sec from exactly 16 bootstrap bytes using fixed HKDF-SHA-256 labels, empty salt and 16-byte outputs. Results have canonical base64url, redacted diagnostics and best-effort owned-buffer erasure. Bootstrap provenance stays with the native caller; derivation grants no authority.

Adds only approved exact hkdf =0.13.0 with defaults disabled. All 152 existing lock tuples and WebSocket exports/pins remain; no store backend, policy allowance or new RustCrypto generation is added.

Commands run and results, from the checkout root:

• cargo test --locked --offline --manifest-path crates/Cargo.toml -p draftyak-transport: 67 passed. The same command with --test sidecar: 7 passed, including both committed anchors and 23 independent Node inputs.
• cargo fmt --all --manifest-path crates/Cargo.toml --check: passed.
• cargo clippy --locked --offline --manifest-path crates/Cargo.toml --workspace --all-targets -- -D warnings: passed with default features.
• cargo-deny --locked --offline --manifest-path crates/Cargo.toml --config crates/deny.toml --workspace --format json --color never check --show-stats --deny warnings: passed under default and --all-features metadata selection. The paired checks adding --audit-compatible-output advisories also passed: four checks, zero errors/warnings, RustSec 4b587f00996ebe5c05a73eaef105ef241e876414 confirmed before and after audit. No all-feature compilation.
• pnpm install --frozen-lockfile --offline --ignore-scripts, pnpm guard --commits origin/m1b..HEAD and git diff --check origin/m1b..HEAD: passed.

Ten semantic guard mutations apply to the unchanged implementation; two current manifest-pin mutations failed at the intended assertions. Restored checks passed. Graph metadata contains 153 packages; HKDF/HMAC requested features are empty, with no optional KDF.

Current CI: full and Windows jobs succeeded, while cheap failed in the unchanged SDK stdio-discovery test. That failure still blocks required/review; its cause and owning disposition are pending. No skipped or failed job is claimed as a pass. DCO remains due at milestone sign-off.

Not run: native stores, a real supervisor or hands-on platform checks. Those belong to later owning items. Erasure excludes borrowed storage, library internals and compiler copies.

Decisions/handoffs: exact HKDF admission only; no additional owner decision for this scope. M1.21 sidecar specification/vectors and M1b.1 are merged; M4 supplies genuine bootstrap provenance later. Claims, confirmation and capabilities are outside this API. Carried minors: none applicable.

Self-check

Only the eight M1b.5 paths change on the merged backlog. Fixed length/algorithm/labels, canonical outputs, typed errors, redaction and live-buffer erasure have positive/refusal controls. Dependency admission adds no advisory or graph exception. Current-head required CI, independent review and actual merge remain pending.