perf: avoid copying mock history in call-count assertions

[exo-egor]

Count-only mock assertions currently rebuild the entire argument history to read its length. Use the underlying mock context's counter through a WeakMap, retaining the existing fallback for Jest-compatible mocks. This makes successful count lookups constant-time and adds no fields to mock objects. The registry lives in its own internal module so the bundler does not mistake mock setup for usage of expect.

A synthetic loop of 5,000 mock calls, each followed by toHaveBeenCalledTimes, improved from 185–200 ms to 20–21 ms on Node 22.22.0 / macOS (about 9× for that loop). These are the medians of seven samples after three warm-up loops, in two separate processes per revision. Whole-suite speedup has not been measured.

Validation:

• Full Node, pure-JavaScript, and bundled suites pass: 147, 146, and 70 suites respectively (FORCE_COLOR=1 for the existing color-dependent snapshots).
• The full 70-suite Hermes run passes, including the mock matcher regressions.
• The new regression fails on the original implementation, reporting 10 call-history reads instead of zero. It also covers bound mocks, clear/reset, aliases, external-compatible mocks, completed/thrown calls, and failed assertions.
• Formatting and ESLint pass for all changed files. Full lint reports an existing @exodus/import/no-extraneous-dependencies error in tests/jest/setup.cjs:9, reproduced on untouched upstream main.

Microbenchmark reproduction

Run this from either checkout with EXODUS_TEST_ENGINE=node:test node --input-type=module:

import { performance } from 'node:perf_hooks'
import { jestfn } from './src/jest.fn.js'
import { expect } from './src/expect.cjs'

function run() {
  const fn = jestfn()
  const start = performance.now()
  for (let i = 1; i <= 5000; i++) {
    fn(i)
    expect(fn).toHaveBeenCalledTimes(i)
  }
  return performance.now() - start
}
for (let i = 0; i < 3; i++) run()
const samples = Array.from({ length: 7 }, run).sort((a, b) => a - b)
console.log({ samples, median: samples[3] })