Compile {{this.#field}} via runtime accessor injection

[NullVoxPopuli-ai-agent]

Summary

Closes the gap on #21375 — templates that read a class's private field
({{this.#foo}}) used to render empty because Glimmer's path walker
resolved the tail segment as a string property access (instance['#foo']),
which never reaches a real private slot.

This change introduces:

1. A strict-mode AST plugin (transform-private-field-access) that
   rewrites every this.#field PathExpression into a call to a synthetic
   free variable — (__ember_priv_field_<name> this) — and records the
   discovered field name on meta.privateFields.
2. A small extension to template()'s implicit-form evaluator that
   declares one (inst) => inst.#<name> accessor per recorded name
   inside the wrapper function it hands to the user's eval. Since the
   user's eval sits inside the class's static block, the #<name>
   syntax parses against the class's private slots; the returned
   closures retain that access for every later render.
3. A lexicalScope short-circuit so the rewritten free variables are
   accepted in strict mode, and a clear error if a template uses
   this.#field without an eval option (no other form can reach
   private slots).

The path is strict-mode-only by design; resolution mode has no
lexical-scope plumbing for the synthetic accessor.

Test plan

• Smoke test added in [BUGFIX] #private field access in class-based components #21375 (private field access > it works)
  passes in classic, embroiderWebpack, and embroiderVite scenarios.
• New integration tests in
  runtime-template-compiler-implicit-test.ts:
  • Can read a \#private` field`
  • Can read multiple \#private` fields and pass them to a helper`
• New unit test in template_test.ts asserting compile + private
  slot reachability.
• Full local pnpm testem ci run — 9311 pass / 17 skip / 0 fail.
• pnpm type-check and pnpm lint:eslint:fix clean.

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