Fix dev-server and test-runner bugs; move formatter options; Rollup-style externals

[dannote]

Fixes a batch of issues found while running Phoenix apps (including the PhoenixVapor demo) on Volt 0.19.4, plus two breaking config changes. Every change has a line in the Unreleased section of the changelog.

Breaking changes

• Formatter options move to .formatter.exs. They are read from the :volt key, where mix format plugins keep their options. config :volt, :format now only holds the build output format, so both can be set. A keyword list under the old key raises an ArgumentError that prints the options to move. The formatting-only :root, :sources and :ignore overrides move to the same key. The installer, guides, examples and Volt's own config follow.

  # .formatter.exs
  [
    plugins: [Volt.Formatter],
    volt: [semi: false, single_quote: true]
  ]

• Externals stay as imports in :esm and :cjs output, as in Rollup, Rolldown, esbuild and Bun. Only IIFE output reads them from globals, and the global names in external: %{"phoenix" => "Phoenix"} apply to IIFE only.

  Format	Output for import { Socket } from "phoenix"
  :iife	const { Socket } = Phoenix; (unchanged)
  :esm	import { Socket } from "phoenix";
  :cjs	require("phoenix")

Dev server and watcher

• Stop the page reloading in a loop when a watched file is rewritten with identical content (LiveView colocated hooks on every code reload). The watcher compares contents before rebuilding.
• Resolve relative watch_ignored patterns from the project directory as well as each watch root.
• Convert local CommonJS/UMD files such as vendor/topbar.js to ES modules in development. .cjs and .cts files, which the dev server did not serve, are converted the same way.
• Load one instance of each pre-bundled dependency. Pre-bundles imported siblings and shared chunks without the ?v= hash, so Vue was loaded twice and component libraries failed to render.
• Explain why a development session is unavailable instead of answering a bare 503, for example when a Mix.Tasks.Volt.Dev watcher runs next to plug Volt.DevServer.
• Log Pre-bundled N vendor package(s) only when packages are bundled, not on every request.
• Stop watchers before the Tailwind processes they depend on. The supervisor's child order stopped the Tailwind registry first, so every Tailwind-enabled watcher crashed on shutdown.

Build

• Skip tsconfig.json path mappings that only point at declaration files.
• Read external globals once in code-split IIFE output; chunks also declared them at the top level of the script.

Test runner and types

• Collect every test generated by test.each and describe.each. Lines were zipped onto tests by position, which dropped tests without a detected line. Line lookup now counts newlines once instead of once per test (about 214 ms to 81 ms on a 1,200-test file).
• Declare import.meta.env in the client types.

Verification

• mix test: 752 passed, 5 skipped, 39 excluded by tag. mix format --check-formatted and mix volt.js.check pass.
• The duplicate-Vue and CommonJS fixes were confirmed against the PhoenixVapor demo in dev (reka-ui page, 10 Playwright interaction steps).
• The new env.d.ts type-checks with tsc alongside the other client declarations.
• The shutdown crash was reproduced by starting a Tailwind watcher session and calling Application.stop(:volt); it stops cleanly after the reorder.
• Not verified in a running app: the reload-loop fix (watcher-level tests only).

Not included

• Version bump and dated changelog section.
• The dev server still rescans sources for bare imports on every request under Phoenix's per-request plug init; only the log was fixed.
• import.meta.glob is still not declared in the client types.