fix(native): keep the composer focused when the iOS keyboard shows - #476

🟢 Confidence 10/10 · safe to merge. Contained to the native host's render tree; the fix rests on Rune's element naming, which only the new Rust test pins and I could not run here.
_{no findings · tests covered · risk medium}

The native page is now always wrapped in the keyboard-padding column, so Rune's element ids stay put as the keyboard moves and the composer keeps focus; the paste handler with nothing focused moves to that wrapper, now the root.

• view always builds the keyboard column, padded by keyboard.max(0.0)
• Paste with no field focused is handled on the wrapper, the page's new parent

Before merge

• Manual · Check the focus loop on a real iOS device: the simulator used the Mac's keyboard, so the on-screen keyboard never appeared

_{85d59ce · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}