[compiler] Don't emit spurious import { c as _c } for discarded functions

[poteto]

What

Codegen registers _c (the memo cache import) as a side effect whenever a function compiles with memo slots. The registration persists on ProgramContext.imports even if the function is later discarded ('use no forget', 'use no memo', lint mode, validation errors). If other applied functions in the file compile to 0 memo slots, the stale import { c as _c } from "react/compiler-runtime"; leaks into the output.

Fix

In applyCompiledFunctions, drop the memo cache import if no applied function uses memo slots. If react/compiler-runtime has no remaining specifiers, drop the module entry too so we don't emit a bare import "react/compiler-runtime";.

Reproducer

use-no-forget-multiple-with-eslint-suppression.js:

import {useRef} from 'react';

const useControllableState = options => {};
function NoopComponent() {}

function Component() {
  'use no forget';
  const ref = useRef(null);
  // eslint-disable-next-line react-hooks/rules-of-hooks
  ref.current = 'bad';
  return <button ref={ref} />;
}

NoopComponent applies with 0 memo slots. Component is opted out, but codegen already registered _c for it. Before:

import { c as _c } from "react/compiler-runtime";
import { useRef } from "react";

After:

import { useRef } from "react";

Prior art

TS counterpart to the Rust port's fix in 7e26eb8. That commit also added no-cache-slots-no-import.js, which codifies the "no memo slots, no import" rule.

Test plan

• yarn snap: 1719/1719 passing
• Snapshot for use-no-forget-multiple-with-eslint-suppression loses its spurious _c import
• no-cache-slots-no-import still passes

[poteto]

@mofeiZ @mvitousek going to merge this one since it's just cleaning up unused import