feat: recursive list/struct decomposition for batch-invariant hashing

CLAUDE.md

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+# Project Instructions
+
+## Formatting
+
+Always run `cargo fmt` before committing. Formatting is enforced in CI via GitHub Actions.
+
+## Test-Driven Development
+
+When implementing new features or fixing bugs:
+
+1. Write tests first that check the desired behavior.
+2. Verify the new tests fail (confirming they catch the issue / check the right thing).
+3. Implement the fix or feature.
+4. Verify all previously failing tests now pass.