fix(julia): add JSON dep — corpus_loader.jl uses JSON.parse (#231)

corpus_loader.jl (landed in #198 saturation+typing) does `using JSON`
+ `JSON.parse(raw)`, but Project.toml had only JSON3. Result: every
train-from-corpus invocation fails with `Package JSON not found`.

Add JSON to deps (vs rewriting JSON.parse → JSON3.read; JSON3 returns
lazy structures and the call-site logic expects JSON.jl semantics).

Discovered when launching the first owner-authorised GNN training run
2026-06-02 (stages 3+4 failed cleanly).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath