- Reformat shard-floor justification as an FNXC:TestInfrastructure comment
(project-standards: AGENTS.md FNXC_LOG convention).
- Clarify that the shard and dashboard-lane 15min floors are not coupled and
may diverge (maintainability: avoid implying an unenforced contract).
- Add a regression-guard test pinning shard.floor=15min and asserting a 525s
derived budget clamps up to the floor, so an accidental revert to the old
5min floor fails loudly (correctness + testing + project-standards).
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