test/ci: wire JS into CI, add embedded-map sync check, drop dead tests

- flake check test-js: runs the full cross-impl test/test against the Node CLI
  (now passes thanks to the package.json type:module fix); JS was untested in CI.
- flake check test-embedded-map-sync: regenerates character_map_embedded.h from
  character_map.txt and fails if it differs from the committed file (kills the
  manual-regen drift footgun the audit flagged).
- remove 4 assertion-less JS 'tests' (test_format/test_performance/test_cross_compat/
  test_web_encoder — zero assertions, not wired, false coverage) and the orphaned
  utils/xxhash32 (referenced nowhere; fuzz_test uses utils/prng).

Author: pmarreck

flake.nix

@@ -296,6 +296,34 @@
             '';
             installPhase = "mkdir -p $out && touch $out/passed";
           };
+
+          test-js = pkgs.stdenv.mkDerivation {
+            name = "test-js";
+            src = ./.;
+            nativeBuildInputs = with pkgs; [ nodejs_24 python3 xxd hexdump ];
+            buildPhase = ''
+              export HOME=$TMPDIR
+              IMPLEMENTATION_TO_TEST=./bin/printable-binary-node.js bash ./test/test
+            '';
+            installPhase = "mkdir -p $out && touch $out/passed";
+          };
+
+          # Guard against a stale generated C header: regenerating from
+          # character_map.txt must reproduce the committed character_map_embedded.h.
+          test-embedded-map-sync = pkgs.stdenv.mkDerivation {
+            name = "test-embedded-map-sync";
+            src = ./.;
+            nativeBuildInputs = with pkgs; [ luajit ];
+            buildPhase = ''
+              cp character_map_embedded.h committed.h
+              luajit utils/generate_embedded_map.lua
+              if ! diff -q committed.h character_map_embedded.h; then
+                echo "character_map_embedded.h is stale; run: luajit utils/generate_embedded_map.lua" >&2
+                exit 1
+              fi
+            '';
+            installPhase = "mkdir -p $out && touch $out/passed";
+          };
         } // pkgs.lib.optionalAttrs pkgs.stdenv.isLinux {
           test-ape = pkgs.stdenvNoCC.mkDerivation {
             name = "test-ape";