Smoother reloads: flash-free stylesheet swap, debounced reload, reload only changed pages

[dannote]

Stacked on #49. Makes dev reloads less disruptive for server-rendered sites (Phoenix pages, Astral), after looking at how Vite and Astro handle the same cases.

Changes

• Stylesheet swap without a flash (as Vite does). The client adds a second <link> for the updated stylesheet and removes the previous one once the new one has loaded or failed, instead of changing href in place.
• Debounced reload (as Vite does). Every reload goes through one pageReload(), so a save that produces several messages reloads once.
• Reload only the pages whose HTML changed. A change in reload_dirs used to reload every open page.

How page revalidation works

Volt does not know which pages a template or content file feeds. Neither Vite nor Astro tracks that either: Vite scopes reloads to the edited .html page, and Astro reloads everything.

1. Each successful HTML response that passes through Volt.DevServer gets an entity tag of its HTML, in the etag header and in data-volt-etag on the client <script>.
2. On a change in a reload directory the watcher broadcasts a document update instead of a full reload.
3. The client requests its own page again with If-None-Match. The server renders it, as it would for a reload, and answers 304 when the HTML is the same.
4. Only another answer reloads the page.

No server state or dependency graph is involved. Pages that already load the dev client (site generators inject it themselves) are tagged on their existing tag, so Astral 0.5.1 works without changes. Pages without the tag, or whose HTML differs on every render, reload as before.

Volt.HMR.document_update/2 sends the update from other packages; Volt.HMR.Document documents the contract for servers that send HTML without going through Volt.DevServer.

Verification

• mix test: 754 passed. Credo, the architecture check and Dialyzer pass locally.
• Browser tests (--include integration, not run in CI): 29 passed, including a new one where editing another page's content does not reload the open page and editing its own content reloads it once.
• Run against a copy of a real Astral site with three pages "open" (simulated by a script doing the conditional request): editing a post reloaded only that post; a whitespace-only edit and a HEEx comment in the layout reloaded nothing; a shared component edit reloaded only the page using it.
• Not run locally: the type-aware JS lint step (tsgolint missing on my machine).

Also in this PR

• The overlay browser fixture still used the pre-0.19.0 overlay API and failed, which stopped the client fixture run before the stylesheet tests. It is rewritten for the current API.

Not included

• The page being edited still does a full reload; patching the DOM in place is a separate design.
• Asset files in a reload directory under the asset root still take the asset path and reload fully.