refactor(ffi-linkage): Phase 9.2d — rename foreign_path → runtime_path (coupled .sx↔.zig↔hook)

The JNI/runtime-class path (Decision 5, Runtime* family). Coordinated across the
hook boundary so the BuildOptions accessor + its registered hook string stay in sync:
- src/: RuntimeClassDecl.foreign_path→runtime_path, splitForeignPath→splitRuntimePath,
  foreignPathToJavaName→runtimePathToJavaName, jni_main_foreign_paths→
  jni_main_runtime_paths, hookJniMainForeignPathAt→hookJniMainRuntimePathAt, and the
  hook string 'BuildOptions.jni_main_foreign_path_at'→'…runtime_path_at'.
- library/: build.sx accessor jni_main_foreign_path_at→jni_main_runtime_path_at +
  bundle.sx call sites + the  local var → runtime_path + a comment.
- specs.md: the accessor name + <foreign_path_with_dots> doc refs.
- Regenerated 37 .ir snapshots: every program importing build declares the renamed
  @BuildOptions.jni_main_runtime_path_at hook stub — symbol-name change only (verified
  the .ir diff is ONLY this rename; reverted orthogonal empty-file normalization).
Suite green (646 corpus / 444 unit, 0 failed).

src/main.zig

@@ -704,7 +704,7 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
         if (e.build_config.manifest_path == null) e.build_config.manifest_path = merged_config.manifest_path;
         if (e.build_config.keystore_path == null) e.build_config.keystore_path = merged_config.keystore_path;
         // `#jni_main` decls flow from the compiler's lowering pass —
-        // pre-rendered Java sources + the foreign_path for each. Build
+        // pre-rendered Java sources + the runtime_path for each. Build
         // two parallel slices since BuildConfig hooks return strings.
         const jni_decls = comp.getJniMainEmissions();
         if (jni_decls.len > 0) {
@@ -722,13 +722,13 @@ fn compileWithTimer(allocator: std.mem.Allocator, io: std.Io, input_path: []cons
             const fps = try allocator.alloc([]const u8, jni_decls.len);
             const srcs = try allocator.alloc([]const u8, jni_decls.len);
             for (jni_decls, 0..) |em, idx| {
-                fps[idx] = em.foreign_path;
+                fps[idx] = em.runtime_path;
                 srcs[idx] = if (lib_name) |ln|
                     try sx.ir.jni_java_emit.injectLoadLibrary(allocator, em.java_source, ln)
                 else
                     em.java_source;
             }
-            e.build_config.jni_main_foreign_paths = fps;
+            e.build_config.jni_main_runtime_paths = fps;
             e.build_config.jni_main_java_sources = srcs;
         }
     }