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
(_getProp) tried instance['#foo'], which never reaches a real
private slot in JS.

This change teaches the property walker about private fields directly,
so the existing path-resolution machinery handles {{this.#foo}} end-to-end:

1. Per-class reader registry in @ember/-internals/metal. A class
   registers a single (instance, fieldName) => unknown reader; the
   reader is constructed inside the class body so its __inst.#name
   syntax is bound at parse time to the class's private slots, and its
   closure preserves that access for any future instance handed to it.
   Lookup walks the receiver's prototype chain — correct for subclasses
   since a parent class's reader can read its own private fields on
   subclass instances.

2. _getProp short-circuit: when the key starts with #, it grabs
   the reader for the receiver's class and delegates. This is the only
   seam in the runtime that needed teaching.

3. Lightweight collect-private-fields AST plugin (no rewrite). It
   walks PathExpressions and writes each #-prefixed tail segment
   into meta.privateFields. The host (template()) wires that bag up
   before precompile and consumes the names afterwards.

4. template() wires up the reader via the user's eval after
   precompile. Because eval sits inside the class's static {} block,
   the __inst.#name syntax in the generated reader binds to the
   class's private slots; the resulting closure is registered against
   the component class.

5. AST formalization in @glimmer/syntax: new isPrivateFieldSegment
   / privateFieldName helpers so consumers don't have to string-match.
   The tail: string[] shape is preserved so existing AST consumers
   keep working.

The explicit scope form does not support {{this.#field}} — its
scope arrow is evaluated outside the class body, so there's no way to
bind #name in it. The implicit (eval) form covers gjs / <template>,
which is what RFC 0921 / content-tag emit.

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.
• Three integration tests in
  runtime-template-compiler-implicit-test.ts: single field,
  multiple fields piped through a helper, and a base-class reader
  read on a subclass instance.
• Two unit tests in template_test.ts: registry registration and
  the no-eval error path.
• Full local pnpm testem ci run — 9313 pass / 17 skip / 0 fail.
• pnpm type-check clean. (pnpm lint:eslint has the same 24
  pre-existing errors in vendored glimmer-vm code as main/this
  PR's base; none of my files have lint errors.)

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