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).

specs.md

@@ -2972,7 +2972,7 @@ back inside the post-link callback.
 | `is_macos() / is_ios() / is_ios_device() / is_ios_simulator() / is_android()` | read | per-target predicates |
 | `framework_count() / framework_at(i)` | read | linker `-framework` names (for `Frameworks/` embed) |
 | `framework_path_count() / framework_path_at(i)` | read | linker `-F` search paths |
-| `jni_main_count() / jni_main_foreign_path_at(i) / jni_main_java_source_at(i)` | read | `#jni_main` emissions for the APK bundler |
+| `jni_main_count() / jni_main_runtime_path_at(i) / jni_main_java_source_at(i)` | read | `#jni_main` emissions for the APK bundler |
 | `asset_dir_count() / asset_dir_src_at(i) / asset_dir_dest_at(i)` | read | iterate registered asset trees |
 
 Returned strings are `""` when unset; integer counts are `0`. Accessors
@@ -3046,7 +3046,7 @@ The Android branch:
 3. **Stage `<apk>.stage/lib/arm64-v8a/<libfoo.so>`** — `copy_file` from the linked output.
 4. **Manifest** — user-supplied via `set_manifest_path()`, or synthesized:
    - `NativeActivity` shape when no `#jni_main` is declared.
-   - `#jni_main` Activity shape with `android:name="<foreign_path_with_dots>"` + `android:hasCode="true"` otherwise.
+   - `#jni_main` Activity shape with `android:name="<runtime_path_with_dots>"` + `android:hasCode="true"` otherwise.
 5. **Compile `#jni_main` Java sources** — write each entry's `java_source` to `<stage>/java/<pkg>/<Cls>.java`, run `javac --release 11 -classpath <android.jar>` to `<stage>/classes/`, run `d8 --release --lib <android.jar> --output <stage>` to produce `<stage>/classes.dex`. `javac` discovered via `$JAVA_HOME/bin/javac` then `command -v javac`.
 6. **`aapt2 link -I <android.jar> --manifest <m> -o <unaligned>`**.
 7. **Append archives** — `zip -q -r <unaligned> lib/`, then `zip -q <unaligned> classes.dex` (if dex was produced), then `zip` each registered asset dir at its `dest` path.