Address review: the fix is about path expression binding, not
the on modifier specifically. Use a plain class + gjs template
to verify that `foo.foo` invoked from a template preserves `this`.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Replace the runtime heuristic (isClassMethod/maybeBindPrototypeMethod in
childRefFor) with a compile-time approach as requested in review.

When the template compiler sees a `this.*` path (GetSymbol with sym=0),
it now emits VM_GET_PROPERTY_BOUND_OP for the last path segment instead
of VM_GET_PROPERTY_OP. The runtime handler for this opcode creates a
reference that auto-binds function values to their parent via .bind(),
with caching for stable identity across revalidations.

Changes:
- New opcode: VM_GET_PROPERTY_BOUND_OP (113) in interfaces, constants,
  and debug metadata
- Opcode compiler: GetSymbol handler emits bound opcode for last segment
  of this.* paths
- Runtime: boundChildRefFor() creates a cached bound reference, skipping
  binding for values with component or modifier managers
- Removed: runtime heuristics from @glimmer/reference (isClassMethod,
  maybeBindPrototypeMethod, etc.)
- Tests: classic Component with class method via {{on "click" this.foo}},
  and nested this.obj.method chain

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Instead of eagerly calling getProp in the bound ref's compute
function (which created parallel tracking and autotracking issues),
the bound ref now returns a stable wrapper function that defers
the property read to call time: parent[path].call(parent, ...args).

This means the property is only tracked through childRefFor's
existing tracking (for typeof check), and the actual method lookup
happens when the function is invoked, not during render.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Pass a bindTransform callback to childRefFor so that binding happens
inside the same ref — same tracking, same caching, same debug labels.
Eliminates the wrapper ComputeRef that was causing autotracking
divergence and identity issues.

The transform wraps function values in a stable callable that defers
property lookup to invocation time. Non-function values (strings,
objects, components, modifiers) pass through unchanged.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- Transform: invalidate wrapper cache when underlying value changes,
  so on/fn modifiers detect callback swaps correctly
- Transform: skip wrapping functions with own enumerable keys, so
  each-in can iterate function properties
- Update fn/on contract tests: the old "no this context" and
  "untouchable context" assertions now verify that this IS preserved
- Update debug-render-tree: use predicate for on-modifier args since
  the callback identity changes with auto-binding
- Remove unused DEBUG import from fn-test.js

Dev: 9140 pass / 0 fail / 17 skip
Prod: 8945 pass / 0 fail / 52 skip

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

CallableFunction's typed .call() method doesn't accept arbitrary
this contexts. Use Reflect.apply with a typeof guard instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- this-binding-test.gjs: use strict mode gjs templates, named classes,
  constructors (not init), no register calls
- fn-test.js: extract context to variable, assert seenThis === context
  to show that this is the defining object (not just non-null)
- Revert debug-render-tree-test.ts to original — the render tree should
  retain original values, this test should not change
- Restore hasDefinitionManager check with detailed comment explaining
  why it's needed (WeakMap-based manager lookups break without it)

The debug-render-tree modifier test still fails (1 failure) because
the bindTransform wrapper changes function identity for arrow functions
passed via this.* paths. This is a design tension: the wrapper is needed
for class methods but changes identity for all functions. Need reviewer
input on the right boundary.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Instead of wrapping function values in a callable (which changes
identity and breaks render tree inspection, modifier WeakMap lookups,
etc.), tag the ref with its parent via a WeakMap. The original value
flows through unchanged — valueForRef returns the original function.

Consumers that invoke callbacks (on modifier, fn helper) check
getBindingParentRef(ref) and bind `this` at invocation time:
- on: callback.bind(parentValue) instead of untouchableContext
- fn: fn.call(parentValue, ...) instead of fn.call(context, ...)

This eliminates:
- hasDefinitionManager check (no identity change = no WeakMap issue)
- Object.keys check (no wrapper = no property loss)
- BOUND_FN_CACHE (no wrapper to cache)
- bindTransform / ChildRefTransform (no value transformation)
- debug-render-tree test changes (original values preserved)

Dev: 9140 pass / 0 fail / 17 skip
Prod: 8945 pass / 0 fail / 52 skip

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Move GetPropertyBound emission into withPath so ALL multi-segment
path reads (this.*, @arg.*, blockParam.*, lexical.*) tag the last
segment's ref with its parent. With the ref-tagging approach this
is safe — the value identity is unchanged, only on/fn check the tag.

This means {{#let this.obj as |o|}} followed by o.method correctly
binds method to obj, matching the behavior of this.obj.method.

Also adds a test for the #let + fn pattern per review request:
  {{#let this.obj as |o|}}
    <button {{on 'click' (fn o.method 'did it')}}>click me</button>
  {{/let}}

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

- on.ts comment: "this.*" → "path read" since binding applies to all
  paths now
- fn-test.ts: assert seenThis has expected property (arg1 === 'foo')
  instead of just non-null, matching reviewer feedback
- expressions.ts: alphabetize imports

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Iterates over an array of Item instances with {{#each}}, passes
item.greet to {{on "click"}} for each, and asserts that each
handler fires with the correct Item as this.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>