chore(ui-scripts): add bin diffing script for occasional tooling change monitoring and testing

.gitignore

@@ -14,6 +14,7 @@ __build__
 .env.test
 .tmp/
 .pnpm-store/
+.transpile-diff/
 
 tsconfig.build.tsbuildinfo
 tsconfig.node.build.tsbuildinfo

docs/contributor-docs/transpile-diff.md

@@ -0,0 +1,66 @@
+---
+title: Comparing Transpiled Output
+category: Contributor Guides
+---
+
+# Comparing Transpiled Output
+
+When migrating build tooling — Babel → SWC, `tsc` → `tsgo`, a Babel preset
+bump, a TypeScript upgrade — you want to know whether the change actually
+altered the emitted code. `transpile-diff` builds two git refs in isolation
+and diffs their `es/`, `lib/` and `types/` output.
+
+```bash
+pnpm run transpile-diff [refA] [refB]
+```
+
+- `refA` defaults to `master`, `refB` to the current branch (`HEAD`).
+- **Any git ref works** — a branch, a tag (`v9.0.0`), or a commit SHA.
+
+```bash
+pnpm run transpile-diff                      # current branch vs master
+pnpm run transpile-diff master my-swc-branch # two branches
+pnpm run transpile-diff v9.0.0 HEAD          # a release vs now
+```
+
+## How it works
+
+For each ref the command:
+
+1. checks it out into a temporary git worktree under `.transpile-diff/`,
+2. runs `pnpm install --frozen-lockfile` then `pnpm run bootstrap` (a clean,
+   full build — so `types/` declarations are included),
+3. snapshots the built output, passing each file through a normalization step
+   that strips the license banner, sourcemap comments and whitespace noise.
+
+It then diffs the two snapshots, prints a `changed / added / removed` summary,
+writes an HTML report to `.transpile-diff/report-<a>-<b>.html` (opened
+automatically), and exits non-zero if anything differs.
+
+Snapshots are cached by commit SHA, so re-running against an unchanged ref
+(e.g. `master`) skips its rebuild.
+
+## Options
+
+| Option        | Default        | Description                                                                                                                                                                                                                         |
+| ------------- | -------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `--modules`   | `es,lib,types` | Which output dirs to compare.                                                                                                                                                                                                       |
+| `--semantic`  | `false`        | Reprint each file with the repo's `dprint` config before diffing, so pure-formatting differences are ignored. **Recommended for Babel→SWC / tsc→tsgo**, where output is never byte-identical but should be semantically equivalent. |
+| `--raw`       | `false`        | Skip normalization entirely — compare raw bytes (banner, whitespace included).                                                                                                                                                      |
+| `--no-frozen` | _(off)_        | Allow lockfile changes when installing. Useful for old refs whose lockfile no longer resolves under the current toolchain.                                                                                                          |
+| `--no-open`   | _(off)_        | Don't open the HTML report automatically.                                                                                                                                                                                           |
+
+### Which mode to use
+
+- **Same compiler, config bump** (e.g. a TypeScript version upgrade): the default normalization is enough — output formatting is stable, so any diff is a real change.
+- **Different compiler** (Babel→SWC, tsc→tsgo): add `--semantic`. Babel and SWC will never produce byte-identical output, but after reprinting through `dprint` the diff reduces to genuine code differences.
+
+`--semantic` uses `dprint`, which is already a ui-scripts dependency — it adds no new packages.
+
+## Notes
+
+- A full `bootstrap` runs per ref, so a cold run takes several minutes — this
+  is meant to be run occasionally, not in CI.
+- Comparing very old commits can fail at the **build** step (lockfile or Node
+  version drift), not the diff step. The command aborts with a clear message
+  identifying which ref failed to build rather than reporting a misleading diff.

package.json

@@ -15,6 +15,7 @@
   ],
   "scripts": {
     "build:themes": "ui-scripts build-themes",
+    "transpile-diff": "ui-scripts transpile-diff",
     "prestart": "pnpm run bootstrap",
     "start": "pnpm --filter docs-app start",
     "start:watch": "pnpm --filter docs-app start:watch",

packages/ui-scripts/lib/commands/index.js

@@ -29,6 +29,7 @@ import deprecate from './deprecate.js'
 import publish from './publish.js'
 import publishPrivate from './publish-private.js'
 import visualDiff from './visual-diff.ts'
+import transpileDiff from './transpile-diff.ts'
 import lint from '../test/lint.js'
 import bundle from '../build/webpack.js'
 import clean from '../build/clean.js'
@@ -46,6 +47,7 @@ export const yargCommands = [
   publish,
   publishPrivate,
   visualDiff,
+  transpileDiff,
   lint,
   bundle,
   clean,