Agent notes — cts/common/

CRITICAL: complete the mandatory AGENTS.md preflight in ../../../../../AGENTS.md before editing here. A change here reaches every schema's checker, so also read the AGENTS.md of each checker under ../*/checker/.

cts_common is what the per-schema checkers share: file-order MCAP reading by schema name, the accumulator model, the envelope and payload checks, verdict aggregation, Mark, the label sidecar, and the viewer's Sample, Track, Skeleton, status-list HTML and submission archive.

• Schema-agnostic. No record type, joint table, profile or check group lives here; tests/test_boundaries.py rejects the names. A shared check carries no group — each schema package subclasses it to set one.
• Same dependency floor as the checkers: mcap and flatbuffers, no isaaccapture. viser only in panel/skeleton.py.
• The oracles import nothing from here. Their quaternion arithmetic is duplicated on purpose; each schema's test_boundaries.py asserts it.
• Changing a shared check changes every schema's verdict. Run each checker's suite.
• The submission archive (panel/bundle.py) takes the schema's parts as arguments: each schema's panel/bundle.py passes the result groups, the frame series and its own package for tool. The archive carries the inputs that cannot be regenerated, above all the labels sidecar. Keep inputs (file hashes, labels, tool.commit) apart from outputs; the packed report is a cache and the recording wins. A missing companion goes in inputs.missing, not an error. It is a zip so report.json reads without decompressing the recording.
• After moving code in or out of here, run pre-commit, not only the tests. Under from __future__ import annotations a name used only in a type hint is never evaluated, so a dropped import passes every test and fails ruff (F821).