Core IR: canonical encoding rules and reference encoder

[flyingrobots]

Scope

Implement the executable canonical encoding rules for Core IR after the normative Core meaning/schema is stable.

Deliverables

• Reference canonical encoder for Core artifacts.
• Decode or validation path sufficient to prove encode/decode stability.
• Deterministic tests for map-order independence, platform independence, mutation sensitivity, and encode/decode stability.
• Tests showing alpha-renaming invariance for local references/SSA ids where applicable.

Non-Goals

• No target lowering.
• No admission.
• No digest freeze until golden byte fixtures are reviewed.