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).

library/modules/build.sx

@@ -84,10 +84,10 @@ BuildOptions :: struct #compiler {
     // `#jni_main #jni_class("path") { ... }` decls collected during
     // lowering. The Android bundler walks `0..jni_main_count()` and
     // for each entry writes a `.java` file at
-    // `<stage>/java/<foreign_path>.java`, compiles via javac + d8, and
+    // `<stage>/java/<runtime_path>.java`, compiles via javac + d8, and
     // bundles the resulting classes.dex into the APK.
     jni_main_count :: (self: BuildOptions) -> i64;
-    jni_main_foreign_path_at :: (self: BuildOptions, i: i64) -> string;
+    jni_main_runtime_path_at :: (self: BuildOptions, i: i64) -> string;
     jni_main_java_source_at :: (self: BuildOptions, i: i64) -> string;
 }