feat(julia-server): wire /gnn/rank to the trained solver

`api/gnn_endpoint.jl` was never included in EchidnaML nor registered
with the HTTP handler — every call to POST /gnn/rank hit the main
server's 404 branch, so the Rust GNN client always fell back through
`reqwest` to cosine similarity regardless of model availability.

Changes
- EchidnaML.jl: include api/gnn_endpoint.jl after api/server.jl so
  GNN_MODEL / GNN_VOCAB globals and route handlers are resolvable.
- api/server.jl: after `init_server_state` loads the NeuralSolver,
  publish it via `GNN_MODEL[]` and compose the HTTP handler with
  `register_gnn_routes!` so /gnn/rank and /gnn/health are served.

Effect: once run_training.jl produces models/neural/final_model and
the server is started against that path, /gnn/rank ranks via the
trained PremiseRanker rather than the cosine fallback.

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

Author: hyperpolymath

src/julia/EchidnaML.jl

@@ -188,6 +188,7 @@ include("training/dataloader.jl")
 include("training/train.jl")
 include("inference/predict.jl")
 include("api/server.jl")
+include("api/gnn_endpoint.jl")
 
 # Initialization
 function __init__()

src/julia/api/server.jl

@@ -265,14 +265,25 @@ function start_api_server(model_path::String;
 
     init_server_state(model_path; cache_size=cache_size, port=port, host=host)
 
+    # Wire the GNN endpoint into the trained solver so /gnn/rank uses it
+    # instead of falling back to cosine similarity.  The solver loaded by
+    # `init_server_state` is a full NeuralSolver (text_encoder + premise_ranker);
+    # publish it as the GNN model and combine the route table.
+    GNN_MODEL[] = SERVER_STATE[].solver
+    GNN_VOCAB[] = SERVER_STATE[].vocabulary
+    @info "GNN endpoint wired to trained solver at $model_path"
+
     @info "Endpoints:"
     @info "  GET  /health"
     @info "  GET  /metrics"
     @info "  GET  /provers"
     @info "  POST /predict"
     @info "  POST /suggest"
+    @info "  POST /gnn/rank"
+    @info "  GET  /gnn/health"
 
-    server = HTTP.serve(handle_request, host, port)
+    combined = register_gnn_routes!(handle_request)
+    server = HTTP.serve(combined, host, port)
     return server
 end