feat(store)!: make json feature additive — repository() returns NeedsCodec unconditionally (#211)

[joeldsouzax]

Summary

Closes #211. Makes the json (and snapshot-json) feature purely additive so that Cargo feature unification can never flip a public signature out from under a crate that didn't ask for it.

Two builder methods previously returned different public types depending on #[cfg]:

• Store::repository() → RepositoryBuilder<S, JsonCodec, A> with json, RepositoryBuilder<S, NeedsCodec, A> without.
• RepositoryBuilder::snapshot_store() → byte-level store + auto-JsonCodec wrap with snapshot-json, pre-composed typed store without.

Under feature unification, a transitive dep enabling json would silently change these signatures for everyone — code written against NeedsCodec / a typed snapshot store would fail to compile on certain dependency graphs. Latent breakage, exactly the kind the 1.0 freeze audit (cross-cutting I4) flags.

Changes

• repository() is now unconditional and always returns RepositoryBuilder<S, NeedsCodec, A>. NeedsCodec::new() drops its #[cfg(not(json))] gate.
• .json() builder method (#[cfg(json)]) — sugar for .codec(JsonCodec::default()). Enabling json only adds this method.
• snapshot_store() split: always-available typed-store form (under snapshot), and a new #[cfg(snapshot-json)] snapshot_store_json() for the byte-level auto-JSON-wrap convenience.
• Migrated all call sites that relied on the hidden JSON default to the explicit surface: fjall-end-to-end example (7), serde_codec_tests (1), snapshot_integration_tests (16), snapshot_tests (3).

Before → after

// events
store.repository::<Order>().build()            // implicit JSON default
store.repository::<Order>().json().build()      // explicit

// snapshots (byte-level store)
store.repository::<A>().snapshot_store(bytes).build()        // implicit wrap
store.repository::<A>().json().snapshot_store_json(bytes).build()  // explicit

The two jsons in the snapshot form are two distinct codecs — one for the event stream, one for the aggregate-state snapshot; both were previously silent defaults.

Verification

• ✅ cargo build -p nexus-store (default features, json off) — repository() compiles as NeedsCodec
• ✅ cargo test -p nexus-store --no-run (json on via dev-dep) — .json() / snapshot_store_json() exercised
• ✅ cargo build -p nexus-example-fjall-end-to-end
• ✅ full nix flake check (clippy, fmt, doc, deny, audit, hakari, nextest) green

Both feature states are covered by the existing CI matrix (empty default features = json off; test dev-dep pulls snapshot-json = json on), so the additivity property is verified without a bespoke test.

Breaking change

Marked feat(store)!. Callers relying on the implicit JSON default must add .json(); snapshot-json users pass byte-level stores to snapshot_store_json() instead of snapshot_store(). Pre-1.0 experimental API.

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