[compiler] avoid outer-scope identifier conflicts in synthesizeName

[sleitor]

Summary

When a component uses an outer-scope binding named $ (e.g. a utility helper function or an imported component alias), the compiler's synthesizeName('$') would not detect the conflict and would generate a memo cache variable also named $, overwriting the outer-scope binding at runtime inside Client Components.

Root cause

Context.synthesizeName checks uniqueIdentifiers (local variable names + referenced globals inside the compiled function) but does not check the Babel program scope for outer-scope bindings. A function or import named $ that is used inside a component but not recursively referenced inside the same function body is invisible to the collision check.

Fix

Extend the collision-avoidance loop in Context.synthesizeName to also call env.programContext.hasReference(validated), which queries the Babel scope for existing bindings (imports, outer declarations). If a conflict is found, the synthesized name is incremented ($0, $1, …) until a unique name is found.

Before fix:

// $ is overwritten — broken in Client Components
import { Stringify as $ } from 'shared-runtime';
function Component({x}) {
  const $ = _c(2); // ← collision!
  ...
}

After fix:

import { Stringify as $ } from 'shared-runtime';
function Component({x}) {
  const $0 = _c(2); // ← no conflict
  ...
}

How did you test this change?

Added two regression test fixtures in compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/:

1. jsx-dollar-sign-component.tsx — outer helper function named $
2. jsx-dollar-sign-component-imported.tsx — $ imported from an external module

All 1721 compiler tests pass (yarn workspace babel-plugin-react-compiler test).

Fixes #36167