feat(s138): deterministic CBOR and COSE_Sign1 with ML-DSA-87

[HackTuah]

Slice 138. Deterministic CBOR and COSE_Sign1 with ML-DSA-87, in the N6 profile, as one boundary with no dependency on the rest of Trinity. It is groundwork: nothing Trinity signs changes, and no statement uses the new format yet. Stacked on #132 (it reuses that slice's :mldsa87_runtime tag and the fips job's mode-off step); retarget to main once #132 merges.

The profile implemented is v0, draft r3, provisional. Every rule may change until a frozen v1 is published with its own vectors, and the pin (Trinity.N6.Profile.pin/0) names the draft and the SHA-384 of its text.

What it does

• The codec (Trinity.N6.CBOR). An encoder that can only produce RFC 8949 section 4.2.1's deterministic encoding, for integers in [-2^63, 2^64-1], tagged byte and text strings (text valid UTF-8 in NFC), arrays, maps with keys 0..2^32-1, and false, true, null. A decoder that reads the input once, stops at the first defect and names it with its byte offset, then re-encodes and compares. Limits: depth 16, 65,536 items, 64 KiB of text, 1 MiB in all, each counted as the moduledoc states. A forged count larger than the input is refused before anything is read.
• Bodies (Trinity.N6.Body). Key 0 is the schema version; a body is checked against its declared schema with no unknown key, and a field declared a set must be sorted by encoded bytes with no duplicate.
• Digests (Trinity.N6.Digest). SHA-384 over foreign bytes exactly as given, as [-43, digest] and a reference carrying the exact length.
• COSE_Sign1 (Trinity.N6.COSE). The protected header is closed to alg -50 (ML-DSA-87), kid and typ, plus CWT claims for a SCITT-renderable typ. kid is the RFC 9679 SHA-384 thumbprint over RFC 9964's key members. ToBeSigned is RFC 9052's, with an empty external_aad. Signing takes the private key as the 32-byte seed or the expanded key. Verification answers the profile's exit classes; a runtime that cannot run ML-DSA-87 answers no trust (5, alg_unavailable), never a crash and never a pass. Both exchange forms, detached and tagged-attached, render from the same three parts.
• A census of every hashing and signing call in lib/, each classified. Four sites re-encode data from elsewhere before hashing it; they are declared there with what it would take to hash the exact bytes, and none changes in this PR.

Vectors

scripts/n6_vectors.exs writes test/fixtures/n6/ (338 files, 67 KB) in the layout the draft gives. Every expected encoding is a hex literal, copied from RFC 8949 Appendix A or built by hand from CBOR heads, so no vector is the encoder's own output. The thumbprint vector uses NIST ACVP's published ML-DSA-87 keyGen case; no key of this project's is published. sha384sum -c SHA384SUMS checks the set from its directory, and a test holds the checked-in set to the generator byte for byte.

Evidence

• Every positive vector encodes and decodes; every negative vector is refused with its code at the top level, in an array and as a map value, and by the one pass alone, never left to the round trip. A property over generated values; each limit at its value and one past it.
• Planted violations, each restored and compared byte for byte afterwards: a decoder that accepts floats turned exactly the nine float and NaN vectors red; one that skips the duplicate-key check turned exactly the two duplicate-key vectors red; a wrong thumbprint byte, a changed vector byte, a verifier that accepts every signature, an unclassified hashing call and a call out of the boundary each turned their own tests red.
• On OTP 28.5.0.8 with OpenSSL 3.5.9, mode off: signing with NIST's test key as a seed and as an expanded key, both verifying; one flipped byte in the header, the payload or the signature is invalid, and a flipped key id is no trust. The fips job's mode-off step runs these.
• An opt-in run (mix test --only n6_corpus): 1,000,000 generated bodies round-tripped, and 13,000,000 mutants from thirteen operators were all refused with their operators' codes (34.6 s).
• Decode time at the limits, median of 20: 1 MiB byte string 38 µs, 65,535-item array 11.1 ms, 32,767-pair map 18.4 ms.

Not in this PR

Receipts keep their slice 024 format; moving them is a later slice. Two-signer carriers, the attribution exit classes (6, 7), the RFC 9995 hash envelope, and any classical algorithm in N6 are outside it.