Write the test suite

[nikki-t]

P1-12 — Write the test suite.

• Write the pytest suite.
• conftest.py builds synthetic .nc fixtures in code (good, missing-var, wrong-dtype, wrong-shape, extra-var, missing-units) so no binaries are committed.
• Cover the reader, both validators, the rules, the parser round-trip, the CLI, and the drift checks.

AC:

• pytest is green.
• Each red path asserts its specific message.

[nikki-t]

Tracked in beads as confluence-contracts-9qw (Phase 1, plan: #5). Sub-tasks and dependency state live there; this issue stays the team-visible record and closes when its PR merges.