fix: restore ES5 output for bundled.js, and assert it

`bundled.js` stopped being ES5 when the webpack build was replaced by tsdown
in #628. It parses at ecmaVersion 2015 but not 5:

    var t=(t,e)=>()=>(e||(t((e={exports:{}}).exports,e),t=null),e.exports)

That is rolldown's `__commonJS` interop helper. `@rolldown/plugin-babel` only
transforms input modules, and rolldown appends the helper afterwards; terser
with `ecma: 5` avoids introducing newer syntax but does not transpile, so the
arrow functions reached the published file. `es5.js` has no CommonJS
dependencies and never gets the helper, which is why only `bundled.js` broke.

The effect is total rather than partial: in an ES5 engine the whole script is
a SyntaxError, so `bundled.js` — the bundle that exists specifically to serve
those engines, polyfills included — does not load at all there.

Lower the emitted chunk with a babel renderChunk pass that runs after bundling
and before terser, so rolldown's own helpers are covered too. Costs 2.8 kB
(+1.6%) on bundled.js; es5.js is unchanged at 34 kB.

The existing guard was a grep for backticks, which this syntax slips straight
past. Replace it with an acorn parse at ecmaVersion 5 over both legacy
bundles, plus a tokeniser check for real template literals (backticks inside
core-js string literals are fine, and 2.14.1 shipped three of them).

Verified by reverting the build fix: `bundled.js parses as ES5` fails, and
passes again once restored. Also confirmed bundled.js loads and parses the
live navigator.userAgent in a real browser.

pnpm-lock.yaml

@@ -42,6 +42,9 @@ importers:
       '@rolldown/plugin-babel':
         specifier: 0.2.3
         version: 0.2.3(@babel/core@7.29.7(supports-color@7.2.0))(rolldown@1.2.0)
+      acorn:
+        specifier: ^8.18.0
+        version: 8.18.0
       ava:
         specifier: ^3.0.0
         version: 3.15.0(supports-color@7.2.0)
@@ -1330,8 +1333,8 @@ packages:
     resolution: {integrity: sha512-HEHNfbars9v4pgpW6SO1KSPkfoS0xVOM/9UzkJltjlsHZmJasxg8aXkuZa7SMf8vKGIBhpUsPluQSqhJFCqebw==}
     engines: {node: '>=0.4.0'}
 
-  acorn@8.17.0:
-    resolution: {integrity: sha512-xRQbDb9BnwDafYNn6Vwl839DYVjqXYb1XVGtWAZ1kcDc6iwAL4hg3B1dZlRiuENFeO2H53gFG3in621AdERVAg==}
+  acorn@8.18.0:
+    resolution: {integrity: sha512-lGq+9yr1/GuAWaVYIHRjvvySG5/4VfKIvC8EWxStPdcDh/Ka7FG3twP6v4d5BkravUilhIAsG4Qj83t02LWUPQ==}
     engines: {node: '>=0.4.0'}
     hasBin: true
 
@@ -5853,15 +5856,15 @@ snapshots:
       mime-types: 3.0.2
       negotiator: 1.0.0
 
-  acorn-jsx@5.3.2(acorn@8.17.0):
+  acorn-jsx@5.3.2(acorn@8.18.0):
     dependencies:
-      acorn: 8.17.0
+      acorn: 8.18.0
 
   acorn-walk@8.3.5:
     dependencies:
-      acorn: 8.17.0
+      acorn: 8.18.0
 
-  acorn@8.17.0: {}
+  acorn@8.18.0: {}
 
   aggregate-error@3.1.0:
     dependencies:
@@ -6006,7 +6009,7 @@ snapshots:
   ava@3.15.0(supports-color@7.2.0):
     dependencies:
       '@concordance/react': 2.0.0
-      acorn: 8.17.0
+      acorn: 8.18.0
       acorn-walk: 8.3.5
       ansi-styles: 5.2.0
       arrgv: 1.0.2
@@ -6993,14 +6996,14 @@ snapshots:
 
   espree@10.4.0:
     dependencies:
-      acorn: 8.17.0
-      acorn-jsx: 5.3.2(acorn@8.17.0)
+      acorn: 8.18.0
+      acorn-jsx: 5.3.2(acorn@8.18.0)
       eslint-visitor-keys: 4.2.1
 
   espree@11.2.0:
     dependencies:
-      acorn: 8.17.0
-      acorn-jsx: 5.3.2(acorn@8.17.0)
+      acorn: 8.18.0
+      acorn-jsx: 5.3.2(acorn@8.18.0)
       eslint-visitor-keys: 5.0.1
 
   esprima@4.0.1: {}
@@ -8915,7 +8918,7 @@ snapshots:
   terser@5.49.0:
     dependencies:
       '@jridgewell/source-map': 0.3.11
-      acorn: 8.17.0
+      acorn: 8.18.0
       commander: 2.20.3
       source-map-support: 0.5.21