iOS: presses are felt (#430)

The page asks the app for a haptic ({t:"haptic", kind}) and the app
plays the phone's own. A light tap for play, pause, flag and pull to
refresh; the system's success for marking read, one item or all; its
warning with an error toast. A browser has no host to ask, and an
older app ignores the message, so there is nothing to gate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/README.md

@@ -72,6 +72,7 @@ Page to app, on `webkit.messageHandlers.ipx`:
 | `{t:"keep", url}` `{t:"unkeep", url}` | keep a file on the phone, or let it go: the page's path for it, `/media/42` or `/media/42/deadair` (#425) |
 | `{t:"clearKept"}` | signing out: everything kept goes |
 | `{t:"sleep", secs}` | pause after `secs`, fading out over the last ten; 0 turns the timer off (#429) |
+| `{t:"haptic", kind}` | a press felt: `tap`, `done` (marked read) or `warn` (an error) (#430) |
 | `{t:"route", x, y, w, h}` | open the AirPlay picker, pointing at the page's button, in the web view's points (#429) |
 
 App to page, as `window.ipxNative.on(...)`: