dx: hot reload should preserve scroll position and form state

[justrach]

Problem

Current hot reload does a full location.reload() on file change. This loses:

• Scroll position
• Form input values
• Modal/dropdown state
• WASM component state

Proposed Solution

Instead of full reload, inject the new HTML via morphdom-style DOM diffing:

1. Server sends changed HTML over SSE
2. Client-side diffing patches only the changed nodes
3. Scroll position, form values, and focus are preserved

Implementation sketch

• SSE sends the re-rendered page body (not a reload signal)
• A tiny (~2KB) client-side diff script patches the DOM
• For WASM components: serialize state before patch, restore after

Complexity

Low-medium. The SSE infrastructure exists. The main work is the DOM diffing logic.