Deferred: mutation helper tooling for Target IR fixtures

[flyingrobots]

Goal

Track optional tooling for reproducible invalid Target IR fixtures used by Echo acceptance tests.

Tracker: #116
Plan: /Users/james/agy-readings/edict/the-plan.md

Deferred because

PR 3 can record mutation-derived invalid fixtures without immediately building a general mutation helper. Tooling becomes valuable once multiple invalid fixture mutations exist.

Future acceptance sketch

• Remove required obstruction reason from a valid Target IR fixture reproducibly.
• Mutate target profile/domain reproducibly.
• Mutate digest fields reproducibly.
• Keep invalid fixtures explicitly mutation-owned, not compiler-produced.

[coderabbitai]

🔗 Related PRs

#67 - Add Echo Target IR lowering [merged]
#108 - Freeze canonical Target IR artifacts [merged]

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