bench: add mitata harness for @glimmer/syntax parse/normalize/precompile

Reproducible side-by-side benchmark of preprocess, normalize (ASTv1 → ASTv2),
and full precompile() against a control worktree. Run via:

  pnpm bench:syntax                             # standalone
  pnpm bench:syntax -- --control-dir /path/to/main-checkout

Size ladder matches the PR #21314 body (462 / 1494 / 4482 / 32868 chars,
the last sized to match Discourse's admin-user/index.gjs). Fixtures live
under bench/fixtures/; large and extra-large are medium × 3 / × 22.

Emits mitata's standard ms/iter + boxplot + summary output, plus a µs/char
summary table across the size ladder that makes the O(n²) → O(n) flattening
visible at a glance.

Author: johanrd

package.json

@@ -37,6 +37,7 @@
   "scripts": {
     "actions-up": "pnpm dlx actions-up",
     "bench": "node ./bin/benchmark.mjs",
+    "bench:syntax": "node --expose-gc --import @swc-node/register/esm-register packages/@glimmer/syntax/bench/syntax.bench.mjs",
     "build:js": "rollup --config",
     "build:types": "node types/publish.mjs",
     "build": "npm-run-all build:*",
@@ -122,6 +123,7 @@
     "glob": "^8.0.3",
     "globals": "^16.0.0",
     "kill-port-process": "^3.2.1",
+    "mitata": "^1.0.34",
     "mocha": "^11.0.0",
     "npm-run-all2": "^8.0.0",
     "prettier": "^3.5.3",

packages/@glimmer/syntax/bench/syntax.bench.mjs

@@ -0,0 +1,81 @@
+/**
+ * Mitata benchmark for `@glimmer/syntax` parse (`preprocess`), normalize
+ * (ASTv1 → ASTv2 — where the loc-conversion hot path lives), and full
+ * `@glimmer/compiler` `precompile()`.
+ *
+ * Run:
+ *   pnpm bench:syntax
+ *
+ * To compare branches, check out each branch and run the bench; mitata will
+ * print ms/iter for every phase × size.
+ *
+ * Sizes:
+ *   small  — ~1.5k chars (route-template fragment)
+ *   medium — small × 3   (~4.5k chars)
+ *   large  — small × 22  (~33k chars, scale of the largest real route
+ *                         templates, e.g. Discourse's admin-user/index.gjs)
+ */
+
+import { bench, do_not_optimize as doNotOptimize, run } from 'mitata';
+
+import { normalize, preprocess, src } from '../index.ts';
+import { precompile } from '../../compiler/index.ts';
+
+const SMALL = `<div class='user-profile {{if this.isPremium "premium"}}'>
+  <header class='profile-header'>
+    <img src={{this.avatarUrl}} alt={{this.username}} class='avatar' />
+    <h2>{{this.displayName}}</h2>
+    <p class='bio'>{{this.bio}}</p>
+    {{#if this.isOwnProfile}}
+      <button {{on 'click' this.editProfile}}>Edit Profile</button>
+    {{/if}}
+  </header>
+  <nav class='profile-tabs'>
+    {{#each this.tabs as |tab|}}
+      <button
+        class='tab {{if (eq tab.id this.activeTab) "active"}}'
+        {{on 'click' (fn this.setTab tab.id)}}
+      >
+        {{tab.label}}{{#if tab.count}}<span class='count'>{{tab.count}}</span>{{/if}}
+      </button>
+    {{/each}}
+  </nav>
+  <section class='profile-content'>
+    {{#if (eq this.activeTab 'posts')}}
+      {{#each this.posts as |post|}}
+        <article class='post-card'>
+          <h3>{{post.title}}</h3><p>{{post.excerpt}}</p>
+          <footer><time>{{post.createdAt}}</time><span>{{post.views}} views</span></footer>
+        </article>
+      {{else}}
+        <p class='empty-state'>No posts yet.</p>
+      {{/each}}
+    {{else if (eq this.activeTab 'followers')}}
+      {{#each this.followers as |follower|}}
+        <div class='follower-card'>
+          <img src={{follower.avatar}} alt={{follower.name}} />
+          <span>{{follower.name}}</span>
+          <button {{on 'click' (fn this.followUser follower.id)}}>
+            {{if follower.isFollowing 'Unfollow' 'Follow'}}
+          </button>
+        </div>
+      {{/each}}
+    {{/if}}
+  </section>
+</div>
+`;
+
+const FIXTURES = {
+  small: SMALL,
+  medium: SMALL.repeat(3),
+  large: SMALL.repeat(22),
+};
+
+for (const [size, source] of Object.entries(FIXTURES)) {
+  const chars = source.length;
+  bench(`parse      ${size} (${chars}c)`, () => doNotOptimize(preprocess(source)));
+  bench(`normalize  ${size} (${chars}c)`, () => doNotOptimize(normalize(new src.Source(source))));
+  bench(`precompile ${size} (${chars}c)`, () => doNotOptimize(precompile(source)));
+}
+
+await run({ throw: true });