fix(build): add train-deploy step, fix Dockerfile raylib build failure

The Dockerfile.hslm-train was running `zig build` which compiled ALL
targets including raylib GUI binaries that fail on Railway (no raylib).
Added `train-deploy` build step that only compiles hslm-train and
hslm-entrypoint. Dockerfile now uses this targeted step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Antigravity Agent

Dockerfile.hslm-train

@@ -20,11 +20,10 @@ ENV PATH="/usr/local/zig-x86_64-linux-0.15.2:${PATH}"
 WORKDIR /app
 COPY . .
 
-# Build hslm-train + hslm-entrypoint (retry 3x for Zig 0.15.x transient build runner crash)
+# Build ONLY training binaries (train-deploy step — no raylib/GUI targets)
 RUN for i in 1 2 3; do \
       echo "[build] attempt $i/3..."; \
-      zig build hslm-train -Doptimize=ReleaseSafe 2>&1 && \
-      zig build -Doptimize=ReleaseFast 2>&1 && break; \
+      zig build train-deploy -Doptimize=ReleaseFast 2>&1 && break; \
       echo "[build] attempt $i failed, retrying..."; \
       sleep 1; \
     done && \

build.zig

@@ -1611,6 +1611,11 @@ pub fn build(b: *std.Build) void {
     });
     b.installArtifact(hslm_entrypoint);
 
+    // train-deploy: build ONLY training binaries (for Railway Dockerfile — no raylib)
+    const train_deploy_step = b.step("train-deploy", "Build hslm-train + hslm-entrypoint for Railway deploy");
+    train_deploy_step.dependOn(&hslm_train.step);
+    train_deploy_step.dependOn(&hslm_entrypoint.step);
+
     // HSLM tests
     const hslm_tests = b.addTest(.{
         .root_module = b.createModule(.{