test: export Arc fixtures from canonical genesis IDs

[littlehand26]

Summary

• replace test: add Arc fixture exporter for Leafage #5 with a fixture exporter branch that descends from the Arc writer genesis-ID fix in fix: encode genesis transaction IDs as bytes32 #6
• retain the existing Arc A1b exporter and provenance hardening as separate commits
• pin the producer baseline to 23d38e7d0cbf54e184faf3751c619f2169b3ed79
• regenerate the Leafage fixture set twice and verify deterministic normalized output

Motivation

The old fixture PR #5 and the genesis-ID fix #6 diverged at 1ff564a. Retargeting #5 would violate the exporter rule that the producer baseline must be an ancestor of the exporter commit. This replacement keeps a linear, auditable history:

#4 writer correctness
└─ #6 genesis bytes32 transaction IDs
   └─ this fixture exporter

Validation

• provenance self-test — passed
• capture-set self-test — passed
• panic-detection self-test — passed
• two independent export_arc_a1b_fixtures runs — 1/1 each
• normalized capture comparison and manifest generation — passed
• generated genesis fixture uses canonical bytes32 transaction IDs and trace transaction IDs
• bash -n scripts/generate-leafage-a1b-fixtures.sh
• Python compile check for build_leafage_a1b_fixtures.py
• git diff --check

Dependency

Stacked on #6. Merge with a merge commit; do not squash/rebase the parent PR, so the producer baseline remains in this branch's ancestry.

Supersedes #5.

[littlehand26]

Closing this obsolete v0.7.3 fixture-exporter PR as part of the approved old-PR cleanup. It is test tooling, not a missing production dependency, and was not merged into the v0.8.0 Writer. The current testing workflow keeps these tools and evidence local. Branch, local artifacts and review history are retained.