Drop dead PURE_INTERNAL_PACKAGES entries

Of the three entries previously in PURE_INTERNAL_PACKAGES, only
`@glimmer/debug` actually contributes to the hello-world bundle
size:

| array contents | hello-world raw | gzip |
| - | - | - |
| `[]` (no callback)                       | 181.12 KB | 57.82 KB |
| `['@glimmer/debug-util']`                 | 181.12 KB | 57.82 KB |
| `['@glimmer/local-debug-flags']`          | 181.12 KB | 57.82 KB |
| `['@glimmer/debug']`                      | 172.99 KB | 55.35 KB |
| all three                                 | 172.99 KB | 55.31 KB |

`@glimmer/debug-util` and `@glimmer/local-debug-flags` are
already inferred pure by rolldown's static analysis (their content
either has no top-level statements that look like side effects, or
gets fully stripped by the LOCAL_DEBUG macro in prod builds).
Re-asserting them adds nothing.

Inlined the array since the callback now checks a single path
prefix. Added a note explaining the measurement so the next
person adding to the list does so with data, not pattern-matching.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: NullVoxPopuli

rollup.config.mjs

@@ -114,27 +114,18 @@ function sharedESMConfig({ input, debugMacrosMode, includePackageMeta = false })
   };
 }
 
-// Tell rollup which source files actually have top-level side effects.
-// Anything matched by `pure` is treated as side-effect-free: unused
-// imports of it can be dropped rather than carried into a downstream
-// shared chunk. The packages here are either explicitly
-// `sideEffects: false` in their own package.json (which gets lost once
-// files are co-bundled into shared chunks across packages) or are pure
-// utility / data-shape packages with no top-level mutation.
+// `@glimmer/debug` declares `sideEffects: false` in its own package.json,
+// but rolldown loses that signal when files from it get co-bundled into
+// shared chunks alongside non-pure code. Re-asserting purity at the
+// chunk-level callback recovers ~8 KB raw on the hello-world bundle by
+// letting unused imports of `@glimmer/debug` content drop out of those
+// chunks. Add other packages here only after measuring an actual
+// reduction.
 function moduleHasSideEffects(id) {
-  if (!id.includes('/packages/')) return true;
-  for (const pkg of PURE_INTERNAL_PACKAGES) {
-    if (id.includes(`/packages/${pkg}/`)) return false;
-  }
+  if (id.includes('/packages/@glimmer/debug/')) return false;
   return true;
 }
 
-const PURE_INTERNAL_PACKAGES = [
-  '@glimmer/debug',
-  '@glimmer/debug-util',
-  '@glimmer/local-debug-flags',
-];
-
 function glimmerSyntaxESM() {
   return {
     onLog: handleRollupWarnings,