Compare page renders in each tab's websocket process

[dannote]

Builds on the page revalidation and opt-in morph from 0.20.0. A page found out what changed by requesting itself again and diffing the answer against its live DOM, which scripts have modified since. That left three gaps: attributes on <html>/<body>, metadata in <head>, and elements owned by client code (an island's props), which could only be handled by reloading.

Idea

Each open page already has a websocket process that lives as long as the page. With a new :document option and :morph, that process keeps the HTML the server last rendered for its page. On a document update it renders the page again and compares the two server renders, then sends the page only what applies to it:

• nothing, when its HTML is the same;
• a reload, when scripts or stylesheets differ or preserved elements were added or removed;
• otherwise the new HTML to patch, plus what the server changed outside the body's contents.

plug Volt.DevServer, document: {MySite, :render_document, []}

The function is called with the page's path, followed by the listed arguments, and returns {:ok, html} or :error.

What the comparison enables

• Root attributes. Only the attributes the server set, changed or removed on <html> and <body> are applied. One a script set, such as a theme, is in neither render and stays.
• Head. Metadata and links (not scripts, stylesheets or the title) that are gone are removed, and new ones added.
• Owned elements. For a preserved element whose attributes the server changed, the client sets them and dispatches a cancelable volt:element-update event. Its owner re-renders and calls preventDefault(); the page reloads only if nothing handles it.
• No request per page per save. Unchanged pages hear nothing; changed pages get the HTML over the socket.

The page starts from the HTML it was served: the dev server keeps it until the page connects, so opening a page costs no extra render. At most 64 pages per session are kept, for pages that never connect. A page that was served other HTML than the server can reproduce falls back to revalidating itself, as in 0.20.0.

Verification

• mix test: 770 passed. mix lint passes.
• Browser tests (--include integration, not run in CI): 33 passed, including an end-to-end test of the push flow: no request from the page, an owned element re-rendered by its owner, root and head changes applied, a script-set attribute kept.
• Tried in a real browser on a copy of an Astral site with the companion Astral branch: changing a post's title and description, and renaming a heading that feeds a Vue island, updated the page with no reload and no request from the page; a second tab on another post received nothing.

Open points

• :document and volt:element-update are new public API between Volt and frameworks.
• The body is still patched with idiomorph from whole HTML; the server does not send patch operations.
• The 64-page cap is arbitrary, and kept HTML is cleared only when the session ends or the cap is reached.
• One unrelated-looking test failed once in an early full-suite run and did not recur in later runs; I could not identify it.