Add agadoo tree-shake regression check

[NullVoxPopuli-ai-agent]

Stacks on top of #21365.

sideEffects: false is a contract that's invisible to reviewers — any module-level mutation (set*Manager(...), Foo.reopenClass(...), _backburner = new Backburner(...)) silently rots the contract. Bundlers will drop these side effects when the symbol they ride along with isn't imported.

This adds a pnpm test:tree-shake script that bundles each currently-pure entry point with rollup as if a consumer imported it for side effects only, and asserts nothing survives. Regression in any known-pure entry fails CI with an actionable message.

What's checked

23 entries that are fully tree-shakeable today:

@ember/-internals/browser-environment    @glimmer/destroyable
@ember/-internals/error-handling          @glimmer/encoder
@ember/-internals/owner                   @glimmer/env
@ember/-internals/utility-types           @glimmer/global-context
@ember/deprecated-features                @glimmer/owner
@ember/destroyable                        @glimmer/util
@ember/owner                              @glimmer/vm
@ember/reactive                           @glimmer/wire-format
@ember/template-compilation               @simple-dom/document
@ember/test                               backburner.js
@ember/version                            dag-map
                                          route-recognizer

The 43 currently-impure entries (@ember/-internals/glimmer, @ember/runloop, @glimmer/runtime, etc.) are omitted because they have known top-level side effects (manager registrations, validator state, backburner instance, etc.) that sideEffects: false is promising bundlers can drop in practice anyway.

Files

• bin/check-tree-shake.mjs — runs agadoo.check() on each known-pure entry, reports failures
• package.json — adds agadoo devDep and pnpm test:tree-shake script
• patches/agadoo.patch — bumps agadoo's bundled acorn from ecmaVersion: 11 (ES2020) to 'latest' so it doesn't choke on private class fields / other ES2022+ syntax that appears in dist/

Caveat

Agadoo bundles by absolute path with no nodeResolve plugin, so it doesn't actually read ember-source/package.json's sideEffects: false. That's a feature here, not a bug: it's measuring "is this file inherently pure?" not "does it tree-shake when consumed as a package." The contract sideEffects: false is making is "no module gains a new top-level side effect that bundlers can't observe through their conservative heuristics," and agadoo enforces exactly that.

Verification

Verified the failure mode by adding globalThis.__OOPS__ = {} at the top level of @ember/destroyable/index.ts. The check correctly fails:

✗ @ember/destroyable — has top-level side effects

1 entry regressed:
  - @ember/destroyable

These packages were previously fully tree-shakeable. Either:
  1. Remove the new top-level side effect (preferred), or
  2. If the side effect is intentional, remove the entry from
     KNOWN_PURE_ENTRIES in bin/check-tree-shake.mjs and update
     ember-source's package.json sideEffects field accordingly.

Test plan

• pnpm test:tree-shake — 23 entries verified pure
• Verified failure mode by adding a deliberate side effect, then reverting

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