fed-prims: Phase C — dag-cbor encode/decode, pure OCaml, RFC 8949 vectors + determinism

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plans/fed-sx-host-primitives.md

@@ -82,7 +82,7 @@ check** → tests → commit → tick box → Progress-log line → push.
 - Tests: sha3-256("") = `a7ffc6f8…0f8434a`; sha3-256("abc") = `3a985da7…11431532`.
 - **Acceptance:** NIST SHA-3 vectors pass; WASM links.
 
-### Phase C — dag-cbor encoder + decoder, pure OCaml
+### Phase C — dag-cbor encoder + decoder, pure OCaml ✅ DONE
 - RFC 8949 deterministic subset (RFC 8742 dag-cbor): unsigned/negative ints,
   byte strings, text strings, arrays, maps with **keys sorted by
   length-then-bytewise**, bool, null, tag 42 (CID link). No floats unless a
@@ -205,6 +205,12 @@ printf '(epoch 1)\n(crypto-sha256 "abc")\n' | \
 
 _Newest first._
 
+- 2026-05-18 — Phase C: pure-OCaml `lib/sx_cbor.ml` (dag-cbor encode/
+  decode), primitives `cbor-encode`/`cbor-decode`. RFC 8949 Appendix-A
+  vectors, length-then-bytewise key sort + order-invariance determinism,
+  decode∘encode round-trip (30 tests). Floats unsupported (raise, no
+  fed-sx shape needs them); tag-42 decode = inner-item passthrough.
+  WASM boot green with new lib module; Erlang 530/530; run_tests +30.
 - 2026-05-18 — Phase B: pure-OCaml `lib/sx_sha3.ml` (Keccak-f[1600] +
   SHA-3 pad, domain 0x06), primitive `crypto-sha3-256`. 4 NIST FIPS 202
   vectors pass (empty/abc/896-bit + 1600-bit 0xa3 multi-block). WASM boot