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/ir/calls.test.zig

@@ -359,7 +359,7 @@ test "plan: foreign-class instance vs static dispatch" {
         .{ .method = .{ .name = "length", .params = &.{}, .param_names = &.{}, .return_type = typeExpr(alloc, "i64"), .is_static = false } },
         .{ .method = .{ .name = "stringWithUTF8String", .params = &.{}, .param_names = &.{}, .return_type = typeExpr(alloc, "i64"), .is_static = true } },
     };
-    var fcd = ast.RuntimeClassDecl{ .name = "NSString", .foreign_path = "NSString", .runtime = .objc_class, .members = &members };
+    var fcd = ast.RuntimeClassDecl{ .name = "NSString", .runtime_path = "NSString", .runtime = .objc_class, .members = &members };
     l.program_index.runtime_class_map.put("NSString", &fcd) catch unreachable;
     _ = module.types.intern(.{ .@"struct" = .{ .name = module.types.internString("NSString"), .fields = &.{} } });