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/jni_descriptor.zig

@@ -169,14 +169,14 @@ pub fn isJniReturnTypeSupported(table: *const types.TypeTable, ret_ty: TypeId) b
     };
 }
 
-/// Encode a (foreign_path, method_name) pair as the JNI-resolved symbol
+/// Encode a (runtime_path, method_name) pair as the JNI-resolved symbol
 /// `Java_<pkg-mangled>_<Class>_sx_1<method-mangled>`. JNI mangling:
 /// `/` → `_`, `_` → `_1`. The `sx_` prefix matches the Java-side
 /// `private native sx_<name>(...)` delegate.
-pub fn jniMangleNativeName(allocator: std.mem.Allocator, foreign_path: []const u8, method_name: []const u8) ![]u8 {
+pub fn jniMangleNativeName(allocator: std.mem.Allocator, runtime_path: []const u8, method_name: []const u8) ![]u8 {
     var buf = std.ArrayList(u8).empty;
     try buf.appendSlice(allocator, "Java_");
-    for (foreign_path) |ch| {
+    for (runtime_path) |ch| {
         if (ch == '/') {
             try buf.append(allocator, '_');
         } else if (ch == '_') {