docs: add PQC crypto posture receipt schema extension

docs/reference/pqc_receipt_schema.md

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+# PQC Crypto Posture Receipt Schema Extension
+
+**Version:** 0.1  
+**Status:** DRAFT  
+**Normativity:** NORMATIVE  
+**Scope:** Interface-only schema extension — no runtime logic changes.
+
+## Purpose
+
+Extends the canonical decision receipt schema (`commit_decision_<hash>.json`) with a `crypto_posture` section that captures algorithm suite, key provenance, and policy alignment for post-quantum cryptography (PQC) governance.
+
+This extension supports deterministic, replayable receipts under DDIL/EMCON conditions and enables CI-level validation of crypto posture compliance.
+
+## Schema: `crypto_posture` Section
+
+The following fields are added as a top-level object within the receipt envelope:
+
+```json
+{
+  "crypto_posture": {
+    "alg_suite": "<string: algorithm suite identifier, e.g. ML-KEM-768, ML-DSA-65, SLH-DSA-128s, or composite>",
+    "key_origin": "<string: key source — e.g. HSM, onboard_kms, ground_authority, external_ca>",
+    "rotation_epoch": "<string: ISO 8601 datetime or epoch marker for key rotation lifecycle>",
+    "policy_hash": "<string: SHA-256 hash of the policy document authorizing this posture>",
+    "toolchain_provenance": "<string: identifier for firmware/toolchain build used>"
+  }
+}
+```
+
+## Field Definitions
+
+| Field | Type | Required | Description |
+|---|---|---|---|
+| `alg_suite` | string | YES | NIST PQC algorithm identifier (ML-KEM, ML-DSA, SLH-DSA) or composite/hybrid identifier |
+| `key_origin` | string | YES | Source of the cryptographic key material |
+| `rotation_epoch` | string | YES | Key lifecycle / rotation epoch marker (ISO 8601) |
+| `policy_hash` | string | YES | SHA-256 hash of the governing policy at decision time |
+| `toolchain_provenance` | string | YES | Build provenance identifier for the signing/encryption toolchain |
+
+## Validation Rules
+
+1. All five fields MUST be present when `crypto_posture` is included in a receipt.
+2. `alg_suite` MUST match an entry in the approved algorithm registry (see conformance vectors).
+3. Unknown or empty `alg_suite` values MUST result in `REFUSE` (fail-closed).
+4. `policy_hash` MUST be a valid SHA-256 hex string (64 characters).
+5. Receipts with `crypto_posture` MUST be canonicalized per RFC 8785 (JCS) when `--canonicalization jcs` is selected.
+
+## Integration with Existing Receipt Envelope
+
+The `crypto_posture` section is an optional extension to the existing `commit_decision_<hash>.json` envelope. When present, it is included alongside the existing fields:
+
+```json
+{
+  "verdict": "ALLOW",
+  "reasons": ["allowlist_match"],
+  "decision_hash": "<sha256>",
+  "request_hash": "<sha256>",
+  "artefact_version": "0.2",
+  "schema_version": "0.2.0",
+  "crypto_posture": { ... }
+}
+```
+
+## Reason Codes (Crypto Posture)
+
+The following reason codes are added to the enumerated registry for crypto-posture decisions:
+
+| Reason Code | Verdict | Description |
+|---|---|---|
+| `crypto_posture_approved` | ALLOW | Posture matches approved policy |
+| `crypto_posture_hybrid_approved` | ALLOW | Hybrid classic+PQC posture approved by policy |
+| `crypto_posture_unknown_suite` | REFUSE | Algorithm suite not in approved registry |
+| `crypto_posture_invalid_signature` | REFUSE | Signature verification failed or algorithm mismatch |
+| `crypto_posture_policy_mismatch` | REFUSE | Posture does not match governing policy hash |
+
+## Notes
+
+- This document describes **interface-level schema only**.
+- No runtime decision logic, gate evaluation, or proprietary algorithms are included.
+- All contributions are independent designs; no proprietary system internals are exposed.