Patch changed pages in place (opt-in morph)

[dannote]

Stacked on #50. After #50, a page whose server-rendered HTML changed still does a full reload. This adds an opt-in mode where the dev client patches the page instead, so scroll position, focus and JavaScript state survive edits to text and classes.

Configuration

config :volt, :server, morph: true
config :volt, :server, morph: [preserve: "[data-island]"]

Value	Meaning
false (default)	A changed page reloads.
true	A changed page is patched in place.
[preserve: selector]	Patched in place, except elements matching the CSS selector, which are left alone with everything inside them.

The setting is emitted as data-volt-morph on the client <script>, which is what the client reads, so a framework can also set it per page.

How it works

The client already has the new HTML at this point, from the revalidation request added in #50. With morph it parses that HTML and patches <body> with idiomorph, vendored into the dev client the same way lit-html is. It then stores the new entity tag and dispatches volt:document-updated on document.

It falls back to a reload when patching would not be safe:

• the scripts the page runs differ (data blocks such as application/json are patched);
• stylesheet links or <style> elements differ;
• the server answers with an error;
• a preserved element was added or removed, or the attributes the server renders for it changed.

Verification

• mix test: 755 passed. mix lint passes in full locally.

• Browser tests (--include integration, not run in CI): 30 passed, with seven new tests for the patching rules and an end-to-end test where a page is patched twice and keeps its state.

• Tried in a real browser on a copy of an Astral site (unmodified Astral 0.5.1, one config line added), on a page with a Vue island:

  Edit	Reloaded	JS state	Scroll	Island
  Add a sentence to the post	no	kept	kept	still mounted and reactive
  Add a class in the layout	no	kept	kept	still mounted
  Add an inline script to the layout	yes (fallback)	lost	reset	remounted

Limits

• Only the contents of <body> and the title are patched. Attributes on <html> and <body> and the rest of <head> stay as they were until the next reload.
• A change to a preserved element's server-rendered attributes (island props) reloads the page; remounting just that element needs a hook in the framework.
• Scripts that attach behaviour to elements do not see newly inserted elements unless they listen for volt:document-updated.
• idiomorph adds about 53 KB unminified to the dev client, whether or not a project opts in.
• Neither Vite nor Astro patches server-rendered HTML in place, so there is no reference behaviour for the edge cases.