Distribution fixtures

agora-plugin-update.json files that must keep loading, and files that must keep being refused, under distribution schema 1.

Kept separate from the manifest fixtures one directory up because they pin a separate contract. How a plugin is published and what a plugin is allowed to do change on different schedules, and an author can alter one without touching the other.

Enforced by crates/agora-plugin-api/tests/compatibility.rs. Adding a fixture after fixing a contract bug is cheap and correct; changing one is a deliberate compatibility decision and should be reviewed as exactly that.

The keys in these files are 0x01… and 0x02… repeated — obviously fake, and never used to verify anything. What is under test is the shape of the file, not cryptography; that is covered by crates/agora-core/src/plugins/updates.rs and crates/agora/tests/signing_round_trip.rs.