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/parser.zig

@@ -1371,7 +1371,7 @@ pub const Parser = struct {
             return self.fail("expected string literal foreign-type path after directive");
         }
         const raw = self.tokenSlice(self.current);
-        const foreign_path = raw[1 .. raw.len - 1];
+        const runtime_path = raw[1 .. raw.len - 1];
         self.advance();
         try self.expect(.r_paren);
 
@@ -1621,7 +1621,7 @@ pub const Parser = struct {
 
         return try self.createNode(start_pos, .{ .runtime_class_decl = .{
             .name = name,
-            .foreign_path = foreign_path,
+            .runtime_path = runtime_path,
             .runtime = runtime,
             .members = try members.toOwnedSlice(self.allocator),
             .is_extern = is_extern_eff,