🧹 refactor: cut the duplicated checks and prose from the tree-shaking work

Acting on ponytail-review. The findings were about verification written twice
and comments restating the CHANGELOG, not about the fixes themselves.

- dist/cjs/package.json had two writers: the build script echoed it, then
  build-bundle.mjs rewrote it three lines later with the sideEffects entry.
  One writer now, the one that knows what belongs in it.
- Dropped the banner-version assert. Same process, same `pkg.version` going in
  and coming out — the "stale bundle" it claimed to catch cannot happen.
- Dropped the `includes('toInstanceSync')` text check. ci.yml imports the flat
  bundle and typeof-checks the export, which is the same claim actually tested.
- Dropped the treeshake case asserting annotations survive minification; the
  build already asserts it. Its one non-duplicated assertion was the floor on
  the expected count, and that gap was real: the build compared flat >= esm, so
  if tsc ever stopped emitting annotations both sides would read 0 and the
  assert would pass on nothing. Folded in as an explicit `expected < 30` check,
  verified by stripping the annotations and watching the build fail.
- Removed the `CEREALE` placeholder from treeshake.test.ts. A template language
  for one variable, with two no-op `.replace('CEREALE', 'unused')` calls left
  behind by it. Interpolated directly.
- Removed `.replace(/\.js$/, '.js')`, which was the identity function.
- Trimmed the comment above the `Symbol.metadata` install from 16 lines to 7,
  and the one above `minifySyntax` from 9 to 3, keeping the parts that are not
  written down anywhere else.

Also synced the CHANGELOG's byte table to the README's. The two had already
diverged in the webpack column — which is the duplication the review warned
about, showing up before anyone edited either on purpose.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SAcqrz3FcadkYr3xG32CjK

CHANGELOG.md

@@ -45,10 +45,10 @@ Measured, minified, across esbuild / rollup / webpack:
 | --- | ---: | ---: | ---: |
 | `flattenErrors` | 394 | 367 | 394 |
 | one decorator | 1,837 | 1,823 | 1,818 |
-| `validateSync` | 3,722 | 3,554 | 3,823 |
-| `toPlainSync` | 7,744 | 7,769 | 7,832 |
-| `toInstanceSync` | 7,900 | 7,942 | 7,956 |
-| a typical DTO | 10,395 | 10,402 | 10,372 |
+| `validateSync` | 3,722 | 3,554 | 3,738 |
+| `toPlainSync` | 7,744 | 7,769 | 7,771 |
+| `toInstanceSync` | 7,900 | 7,942 | 7,944 |
+| a typical DTO | 10,395 | 10,402 | 10,360 |
 | everything | 26,266 | 25,671 | 26,879 |
 
 The serializer and deserializer drop independently. The validator is kept by both mapping