Write a canonical encoder + Core digest explainer

[flyingrobots]

Audit refs: AUD-2026-06-28-V02 §2.3

crates/edict-syntax/src/canonical.rs (905 LOC: encode_canonical_cbor, digest_core_module, the edict.canonical-cbor/v1 framing and edict.core.module/v1 domain-separated SHA-256) is the byte/hash foundation everything will be admitted against, but has no prose walkthrough of the encoding rules, domain-separation rationale, or the "meaning freezes before bytes; bytes freeze before hashes" discipline.

Scope

• Add a dedicated explainer (e.g. under docs/ or the core-ir topic shelf) covering the encoding rules, domain separation, and change discipline, cross-linked to the golden fixtures.

Acceptance criteria

• Explainer added and linked; consistent with canonical.rs and the core-ir shelf; markdownlint + cargo xtask verify green.
• Update docs/audit/2026-06-28_documentation-quality.md §2.3: add ✅ Addressed (YYYY-MM-DD, #PR).

[coderabbitai]

🔗 Related PRs

#14 - Update README implementation status [merged]
#32 - docs: add workflow standards [merged]
#37 - Add v1 lowerability and target-profile conformance checks [merged]
#41 - Add Gate C admission boundary checks [merged]
#67 - Add Echo Target IR lowering [merged]

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