First tests #3

This repository has no tests.

• A fixture test against the worked example in the schema repo, so the reader and the
  schema cannot drift apart silently.
• A decode test against a real exported run, once one exists.
• A negative test per refusal the reader implements: unknown major version, mismatched
  connectivity_sha, a missing required block, an LZ4-compressed file.

The last one matters more than it looks: an LZ4 run fails inside arrow-js with an error
that does not say "this file is compressed", and that is the failure a new user will hit
first.

Done when pnpm test runs in CI on every push.

Depends on #1. The fixture half can land immediately after it; the real-run half needs OpenSourceAWE/SymbolicAWEModels.jl#319. Tracked by OpenSourceAWE/symawe.com#1.