rollup: mark @glimmer/debug{,-util,-flags} as side-effect-free in treeshake

Each of these packages already declares \`sideEffects: false\` in its
own \`package.json\`, but that declaration is lost the moment rollup
bundles their source files into shared chunks alongside other-package
runtime code. Once that happens, downstream bundlers (vite/rolldown
consuming ember-source's dist/prod) load the chunk for any runtime
symbol from another package and end up evaluating the debug top-level
(e.g. the \`STYLES\` const, \`LogFragmentBuffer\` class definitions,
ANNOTATION_STYLES array) — even though every actual call site is
behind \`if (LOCAL_DEBUG) {…}\` and gets DCE'd by Babel.

Add a \`treeshake.moduleSideEffects\` callback to the shared ESM
rollup config that explicitly marks files under those three packages
as pure. Anything else stays default (conservative).

Result: the \`render/styles.ts\` / \`render/buffer.ts\` /
\`render/fragment.ts\` content stops getting bundled into shared chunks
like \`curried-*.js\` at the dist-build stage. Downstream bundlers see
the chunks without any debug top-level code at all.

| | hello-world raw | hello-world gzip | classic v2-app raw | classic v2-app gzip |
| - | - | - | - | - |
| before | 133.40 KB | 42.66 KB | 317.74 KB | 98.77 KB |
| after  | 131.27 KB | 42.06 KB | 309.68 KB | 96.28 KB |

Verified: \`pnpm lint\`, \`pnpm type-check:internals\`, both
v1/v2-app-template smoke-test \`pnpm test\` pass 1/1, hello-world
shrinks across the board.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

Author: NullVoxPopuli

rollup.config.mjs

@@ -100,6 +100,9 @@ function sharedESMConfig({ input, debugMacrosMode, includePackageMeta = false })
   return {
     onLog: handleRollupWarnings,
     input,
+    treeshake: {
+      moduleSideEffects: moduleHasSideEffects,
+    },
     output: {
       format: 'es',
       dir: outputDir,
@@ -111,6 +114,26 @@ function sharedESMConfig({ input, debugMacrosMode, includePackageMeta = false })
   };
 }
 
+// Tell rollup which source files actually have top-level side effects.
+// Anything not on this list is treated as side-effect-free, so unused
+// imports of it can be elided rather than carried into a downstream
+// shared chunk. By default rollup is conservative (`true`) — most
+// internal files don't have a `sideEffects: false` package.json visible
+// at the right boundary.
+function moduleHasSideEffects(id) {
+  // External (node_modules) — leave default behavior.
+  if (!id.includes('/packages/')) return true;
+  // Files under `@glimmer/debug` are pure debug helpers; the package
+  // itself declares `sideEffects: false`, but that gets lost when files
+  // are bundled into shared chunks across packages. Force-mark them as
+  // pure here so unused imports are dropped at the chunk boundary.
+  if (id.includes('/packages/@glimmer/debug/')) return false;
+  if (id.includes('/packages/@glimmer/debug-util/')) return false;
+  if (id.includes('/packages/@glimmer/local-debug-flags/')) return false;
+  // Default: assume internal files may have side effects.
+  return true;
+}
+
 function glimmerSyntaxESM() {
   return {
     onLog: handleRollupWarnings,