[younes-io]

#[test] now records where each test is defined, and discover_case carries it:

{"event":"discover_case","name":"check_toph","source":{"path":"crates/libtest2/examples/simple.rs","start_line":11,"start_col":4,"end_line":11,"end_col":14}}

• the location is the fn name (same as upstream libtest), not the #[test] attribute line
• source is just path plus optional line/col fields, so a fixture can send only {"path": ...}, and byte offsets can be added later as more optional fields without breaking anything
• how it works: a small hidden proc macro moves file!()/line!()/column!() onto the fn name's location, so it still builds on MSRV 1.74
• mimic / FnCase still report nothing for now, that can come later
• known gap: the json schema can't say "all four line fields or none", nesting them under lines would fix that if you prefer it

two commits: json + harness plumbing first (no output change), then the macro that fills it in (snapshots update).

Refs #153, #72

disclosure: written with AI assistance; reviewed and tested by me.