Canonical Target IR bytes + digest (reviewed goldens), then integrate into bundle assembly #105

Context: v0.9 shipped deterministic in-memory Echo/git-warp Target IR review artifacts but deliberately deferred canonical Target IR bytes, digests, and reviewed golden artifacts (a non-goal). v0.11 contract-bundle assembly therefore accepts targetIrDigest as a supplied digest-locked reference, not a value it computes — see docs/design/contract-bundle-assembly-v0.11.md.

This issue tracks closing that gap on the target-IR track (not under bundle pressure), mirroring the discipline the Core path already followed (executable canonical encoder → reviewed bytes → exact digest fixture → regeneration command → hash lock).

Scope

• A canonical CBOR/value model for the supported Target IR artifacts (echo.span-ir/v1, gitwarp.commit-reducer-ir/v1).
• A digest_target_ir_artifact function reusing the edict.digest/v1 domain-separated framing.
• Checked-in reviewed Target IR byte + digest golden fixtures with an xtask check/regenerate path.
• Then: integrate into bundle assembly so targetIrDigest can be computed from a real Target IR artifact (replacing the supplied reference), and upgrade the v0.11 bundle test matrix from "targetIrDigest reference changed" to "target IR bytes rehashed".

Acceptance criteria

• Canonical Target IR encoder + digest + reviewed goldens land with their own RED/GREEN evidence.
• Bundle assembly can compute targetIrDigest from a real artifact; the v0.11 design doc's "Non-claim" about target-IR byte tamper detection is updated/removed.