fix(ctef-matrix): confirm urn:mycelium:trail row #2 — 3 byte-match vectors

[giskard09]

What this PR does

Fills the `urn:mycelium:trail` placeholder (row #2) in the CTEF v0.3.3
cross-extension matrix.

Fixture set

`giskard09/argentum-core/examples/conformance/`
https://github.com/giskard09/argentum-core/tree/main/examples/conformance

3 vectors — each with preimage fields, computed `action_ref`, full CTEF
envelope, `canonical_bytes_hex`, and `canonical_sha256`:

Vector	action_type	agent_id
`nexus-oracle-signal`	`oracle.signal`	`nexus-agent-xa12.onrender.com`
`giskard-self-enter-oasis`	`enter_oasis`	`giskard-self`
`pioneer-negotiation-accept`	`negotiation.accept`	`pioneer-agent-001`

Vector 1 `action_ref` matches the canonical example in
`docs/spec/action-ref.md`
byte-for-byte — cross-reference anchor is deterministic.

Substrate filter

• RFC 8785 JCS canonicalization ✓
• lowercase-hex SHA-256 ✓
• Byte-match reproducible across implementations ✓

Reproduce in Python:

import hashlib, json
def jcs(obj): return json.dumps(obj, separators=(',',':'), sort_keys=True, ensure_ascii=False)
preimage = {"action_type": "oracle.signal", "agent_id": "nexus-agent-xa12.onrender.com", "scope": "BTC", "timestamp": "2025-05-18T11:40:31.000Z"}
action_ref = hashlib.sha256(jcs(preimage).encode()).hexdigest()
# fdd7f810499f06be24355ca8e2bfb8c4b965cc80c838f41fa074683443d89f5a

Change to the matrix

Row #2 status: 🟡 PLACEHOLDER → ✓ CONFIRMED — andysalvo 4/4 + AlgoVoi 4/4 + 3 vectors byte-match verified (JCS RFC 8785 + SHA-256)

Fixture link: updated from bare path to live GitHub link.
Canonical spec: `docs/spec/action-ref.md`.