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_java_emit.test.zig

@@ -42,7 +42,7 @@ test "rejects non-main decl" {
     const a = std.testing.allocator;
     const fcd: ast.RuntimeClassDecl = .{
         .name = "Foo",
-        .foreign_path = "co/example/Foo",
+        .runtime_path = "co/example/Foo",
         .runtime = .jni_class,
         .is_main = false, // ← not main
     };
@@ -73,7 +73,7 @@ test "void onCreate(Bundle) with default Activity superclass" {
     } };
     const fcd: ast.RuntimeClassDecl = .{
         .name = "SxApp",
-        .foreign_path = "co/swipelab/sx_runtime/SxNativeActivity",
+        .runtime_path = "co/swipelab/sx_runtime/SxNativeActivity",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{member},
@@ -116,7 +116,7 @@ test "primitive params" {
     } };
     const fcd: ast.RuntimeClassDecl = .{
         .name = "Sx",
-        .foreign_path = "co/sample/Sx",
+        .runtime_path = "co/sample/Sx",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{member},
@@ -157,7 +157,7 @@ test "declaration-only methods are skipped" {
 
     const fcd: ast.RuntimeClassDecl = .{
         .name = "Sx",
-        .foreign_path = "co/example/Sx",
+        .runtime_path = "co/example/Sx",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{ bodied, decl_only },
@@ -194,7 +194,7 @@ test "#extends Alias resolves through class registry" {
 
     const fcd: ast.RuntimeClassDecl = .{
         .name = "Sx",
-        .foreign_path = "co/example/Sx",
+        .runtime_path = "co/example/Sx",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{ extends_member, method_member },
@@ -204,7 +204,7 @@ test "#extends Alias resolves through class registry" {
     try std.testing.expect(std.mem.indexOf(u8, out, "extends co.example.MyParentActivity") != null);
 }
 
-test "default-package class (no slash in foreign_path)" {
+test "default-package class (no slash in runtime_path)" {
     const a = std.testing.allocator;
     var arena = std.heap.ArenaAllocator.init(a);
     defer arena.deinit();
@@ -222,7 +222,7 @@ test "default-package class (no slash in foreign_path)" {
     } };
     const fcd: ast.RuntimeClassDecl = .{
         .name = "Sx",
-        .foreign_path = "SxNoPackage",
+        .runtime_path = "SxNoPackage",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{member},
@@ -253,7 +253,7 @@ test "lib_name renders System.loadLibrary static init block" {
 
     const fcd: ast.RuntimeClassDecl = .{
         .name = "SxApp",
-        .foreign_path = "co/example/SxApp",
+        .runtime_path = "co/example/SxApp",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{method},
@@ -296,7 +296,7 @@ test "field declarations render as private Java fields" {
 
     const fcd: ast.RuntimeClassDecl = .{
         .name = "SxApp",
-        .foreign_path = "co/example/SxApp",
+        .runtime_path = "co/example/SxApp",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{ view_field, w_field, method },
@@ -334,7 +334,7 @@ test "#implements clauses on the class header" {
 
     const fcd: ast.RuntimeClassDecl = .{
         .name = "SxApp",
-        .foreign_path = "co/example/SxApp",
+        .runtime_path = "co/example/SxApp",
         .runtime = .jni_class,
         .is_main = true,
         .members = &.{ impl_a, impl_b, method },