ADR-0002: JSON-first index format

Status

Accepted (implemented since Phase 1).

Context

concepts/index-format.md commits to the index format being "a spec, not a library dependency" — a Python or Java producer should be able to emit a conforming index with standard-library tooling alone, proven by the two independent reference generators in spec/examples/.

Decision

JSON is the format every shard type supports. Every shard type uses the same JSON encoding, making the index easy to produce and inspect with standard tooling. Facet, synonym, and pins shards follow the same JSON convention.

Alternatives Considered

• A non-JSON format: rejected — it would break the "no library buy-in, a Python script can produce this" goal, since a from-scratch custom codec is a much higher bar for an independent producer than JSON.

Consequences

• JSON is trivial for independent producers to emit — any language with a JSON library can generate a conforming index.
• JSON is easy to inspect, debug, and validate with standard tooling.
• Native JSON.parse is well-optimized in all modern runtimes, keeping client-side decode costs low.