infra: local contour — pre-push gate:small by default, sandbox bootstrap, test:chunk

- .githooks/pre-push + scripts/pre-push-gate.mjs --hook: gate:small runs by
  default for issue/* branches with an executable diff; C/D-only diffs and
  non-issue refs are skipped, HP_PREPUSH_GATE=0 turns it off, =1 forces it.
  A non-HEAD push with an executable diff is rejected (the gate checks the
  working tree). The gate runs with every GIT_* variable removed: git gives
  hooks GIT_DIR (and GIT_CONFIG_PARAMETERS with the pusher's -c), and unit
  tests that `git init` temporary repositories otherwise write into the real
  one — seen on the first live run. The manual #343 mode is unchanged.
- scripts/sandbox-bootstrap.sh: idempotent worktree / deps / chromium (npm
  @sparticuz/chromium@152.0.0 + Playwright shims) / bundle / check steps,
  no owner paths or credentials.
- scripts/test-chunk.mjs, npm run test:chunk -- N/M: round-robin over
  test/*.test.mjs sorted by code point.
- Tests, seven mutants, docs/DEVELOPMENT.md «Локальный контур за 5 минут»,
  docs/TESTING.md. package.json changed → bundle rebuilt (fingerprint input).

Issue: #633
User-Visible: no

scripts/check-inputs.mjs

@@ -68,7 +68,6 @@ export const NOT_AN_INPUT = [
   ['demo/downgrade_open_passage.mjs', 'ручной инструмент миграции фикстур'],
   ['scripts/dev/**', 'локальные утилиты разработчика'],
   ['scripts/install-hooks.mjs', 'установка git-хуков при npm ci'],
-  ['scripts/pre-push-gate.mjs', 'локальный pre-push набор (HP_PREPUSH_GATE), в CI не исполняется'],
   ['scripts/golden-accept.mjs', 'приёмка эталонов человеком, после прогона (#344)'],
   ['scripts/golden-container.mjs', 'локальная съёмка в пиновом образе (#334), ручной запуск'],
   ['scripts/inventory.mjs', 'отчёт для аудита, не гейт'],