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/ast.zig

@@ -885,7 +885,7 @@ pub const RuntimeClassMember = union(enum) {
 
 pub const RuntimeClassDecl = struct {
     name: []const u8, // sx-side alias (left of `::`)
-    foreign_path: []const u8, // directive arg: "java/path/Foo" / "NSString" / "Foundation.URL"
+    runtime_path: []const u8, // directive arg: "java/path/Foo" / "NSString" / "Foundation.URL"
     runtime: RuntimeKind,
     members: []const RuntimeClassMember = &.{},
     is_extern: bool = false, // `#objc_class(…) extern` — class is provided by the runtime; we only reference it (vs `export`, which defines + registers a new sx class)