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/target.zig

@@ -9,9 +9,9 @@ const c = llvm.c;
 /// compiles via `javac`, dexes via `d8`, and bundles the resulting
 /// `classes.dex` into the APK.
 pub const JniMainEmission = struct {
-    /// foreign_path of the source decl (e.g. "co/swipelab/sxmain/SxApp").
+    /// runtime_path of the source decl (e.g. "co/swipelab/sxmain/SxApp").
     /// Splits into package + class name for `<stage>/java/<pkg>/<Class>.java`.
-    foreign_path: []const u8,
+    runtime_path: []const u8,
     /// Pre-rendered Java source bytes (from `jni_java_emit.emitJavaSource`).
     java_source: []const u8,
 };