fix(control-plane): give the snapshot schema version one owner

[zzylol]

The drift

The workload-snapshot schema version was restated at every site that produces or consumes a snapshot: the backend's check, discover_snapshot.py, calibrate.py, planned_run.py, two shipped example snapshots, and their tests. Nothing tied those declarations together.

They drifted. On the #728 stack, two tools still overwrote the version with a stale literal 2 while the backend had moved to 3, so a snapshot built from a current template was relabelled to an old version and then rejected by the very backend that ships that template:

Snapshot("unsupported workload snapshot version 2; only version 3 is supported")

Nothing failed at the point the wrong version was written. It failed later, in an unrelated test, with a message about the version rather than about the producer that wrote it.

The change

WORKLOAD_SNAPSHOT_VERSION is declared once, beside the field it governs, and used for both the check and its message.

Producers are written in Rust, Python and JSON and cannot share a constant, so a test ties them together instead (control_plane/tests/snapshot_version.rs):

• every shipped example snapshot declares that version;
• every listed tool rejects anything else;
• no tool writes snapshot_version. A tool changes a snapshot's content, not its schema, so restating the version is exactly how the literal went stale.

Bumping the schema is now one edit plus whatever the test reports.

Validation

Mutation-checked rather than merely green:

Mutation	Result
Constant raised to 4	both tests FAIL
snapshot["snapshot_version"] = reintroduced into a tool	FAILS, naming that tool
Reverted	2 passed

cargo test -p control_plane --lib: 433 passed. cargo fmt --check clean, cargo clippy --all-targets with -D warnings clean.

Related

Third instance this week of an input contract moving while a hand-written producer of that input stayed behind, after the two dataset_identity sites. The first two had a compiler backstop; a version number is a plain integer and had none, which is how this one reached CI.

🤖 Generated with Claude Code