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/core.zig

@@ -362,10 +362,10 @@ pub const Compilation = struct {
     /// Walk `lowering.program_index.runtime_class_map` and render Java sources for every
     /// `#jni_main #jni_class("...")` declaration. Renders happen here so the
     /// AST + class-registry snapshot stay confined to the lowering pass; the
-    /// downstream APK pipeline only needs `{foreign_path, java_source}` pairs.
+    /// downstream APK pipeline only needs `{runtime_path, java_source}` pairs.
     fn collectJniMainEmissions(self: *Compilation, lowering: *ir.Lowering) !void {
         // `runtime_class_map` registers each decl under bare + qualified names —
-        // dedupe by foreign_path so a single decl emits one .java.
+        // dedupe by runtime_path so a single decl emits one .java.
         var seen = std.StringHashMap(void).init(self.allocator);
         defer seen.deinit();
 
@@ -375,7 +375,7 @@ pub const Compilation = struct {
         defer registry.deinit();
         var it_reg = lowering.program_index.runtime_class_map.iterator();
         while (it_reg.next()) |entry| {
-            try registry.put(entry.key_ptr.*, entry.value_ptr.*.foreign_path);
+            try registry.put(entry.key_ptr.*, entry.value_ptr.*.runtime_path);
         }
 
         // Derive the `System.loadLibrary` argument from the `-o` basename
@@ -390,15 +390,15 @@ pub const Compilation = struct {
             if (!fcd.is_main) continue;
             if (fcd.is_extern) continue;
             if (fcd.runtime != .jni_class) continue;
-            if (seen.contains(fcd.foreign_path)) continue;
-            try seen.put(fcd.foreign_path, {});
+            if (seen.contains(fcd.runtime_path)) continue;
+            try seen.put(fcd.runtime_path, {});
 
             const java_source = try ir.jni_java_emit.emitJavaSource(self.allocator, fcd, .{
                 .classes = &registry,
                 .lib_name = lib_name,
             });
             try self.lowering_jni_main_decls.append(self.allocator, .{
-                .foreign_path = try self.allocator.dupe(u8, fcd.foreign_path),
+                .runtime_path = try self.allocator.dupe(u8, fcd.runtime_path),
                 .java_source = java_source,
             });
         }