Reported by a downstream (Langust) agent after the 1.7.184 RowMenu JS
fix didn't reach the browser even after bumping the dependency: the
core phoenix_kit.js hooks file was only ever vendored into a host app
by a one-shot File.cp in mix phoenix_kit.install/update, unlike
phoenix_kit_modules.js which the :phoenix_kit_js_sources compiler
already regenerates on every mix compile. A deploy that wipes
priv/static without re-running phoenix_kit.update (most CI/CD
pipelines) ships a stale or missing phoenix_kit.js with no error.

- Extend the :phoenix_kit_js_sources compiler to also vendor
  phoenix_kit.js on every mix compile, self-healing after any
  priv/static wipe.
- JsIntegration.update_js_file/0 now raises loudly on failure and
  verifies the destination file post-copy, instead of swallowing
  errors into a return value its only caller never checked.
- mix phoenix_kit.assets.rebuild now refreshes phoenix_kit.js too.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>