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

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

src/julia/Manifest.toml

@@ -8,6 +8,7 @@ Flux = "587475ba-b771-5e3f-ad9e-33799f191a9c"
 GraphNeuralNetworks = "cffab07f-9bc2-4db1-8861-388f63bf7694"
 Graphs = "86223c79-3864-5bf0-83f7-82e725a168b6"
 HTTP = "cd3eb016-35fb-5094-929b-558a96fad6f3"
+JSON = "682c06a0-de6a-54ab-a142-c8b1cf79cde6"
 JSON3 = "0f8b85d8-7281-11e9-16c2-39a750bddbf1"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 Logging = "56ddb016-857b-54e1-b83d-db4d58db5568"