feat(client): 3-finger swipe toggles the on-screen keyboard mid-stream

iOS + Android: a three-finger vertical swipe up/down summons/dismisses the
device soft keyboard while streaming (trackpad + pointer modes). Mobile scroll
is now exactly two fingers so it never collides with the 3+-finger gesture
(3+ only fell into the old `>= 2` scroll path by accident).

Android: a TYPE_NULL KeyCaptureView plus IME meta-shift wrapping feeds key
events through. iOS: UIKeyInput plus a SoftKeyMap char->VK table with a
GCKeyboard dup gate so a hardware keyboard and the soft keyboard don't
double-emit.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/pf-presenter/src/touch.rs

@@ -13,7 +13,9 @@
 //!
 //! Shared gestures: tap = left click · two-finger tap = right click · two-finger drag =
 //! scroll · tap-then-press-and-drag = held left drag · three-finger tap = cycle the stats
-//! overlay tier.
+//! overlay tier. (The Android/Apple twins additionally map a three-finger vertical SWIPE to
+//! their local soft keyboard and gate scroll to exactly two fingers for it; SDL builds have
+//! no soft keyboard to summon, so here 2+ fingers scroll.)
 //!
 //! Unlike the Android/Apple hosts (which hand the engine a whole event's worth of changed
 //! touches at once), SDL delivers ONE finger transition per event, so this is a strictly