[addhumal]

Closes #178.

What it does

• Adds TouchMode (Direct Touch vs Trackpad, saved in Sill.touchMode, default Direct Touch) to PointerPresence.swift and Settings › This iPad / This iPhone.
• When Trackpad is selected, overlays TrackpadView on the stream picture pane and enables the virtual cursor sprite (TouchMode.showsPointer(isPortrait:mode:)) in both landscape and portrait so dragging anywhere on the picture moves the pointer relatively, tapping clicks at the pointer, pressing and holding drags, and two fingers scroll.
• The tour's picture card follows the mode (TourFeatures.trackpadMode): in Trackpad mode it is "The Picture Is a Trackpad" (drag to move the pointer, tap to click, two fingers to right-click and to scroll, hold then drag, and the three-finger swipe row as before), without direct touch's rows.
• The iPad's own trackpad or mouse over the picture in Trackpad mode: its scroll and hover fall through to the picture as in Direct Touch (TrackpadSurface.point(inside:with:)).
• Changing the mode gives back the whole picture: a zoom made before Trackpad mode couldn't be pinched out under it.

Known

• In Trackpad mode the trackpad lies over the picture, so the picture's pinch-to-zoom (Stream: pinch-to-zoom on the picture (1×–5×, centred on the pinch) (#65) #196) and the Apple Pencil as a mouse don't reach it; Direct Touch keeps both.
• The iPad's own trackpad's or mouse's click over the picture in Trackpad mode is the trackpad's (at Sill's pointer, not where iPadOS's pointer is).

Why

• On small screens or dense UIs in landscape, direct touch can be hard to aim accurately (Option to switch between trackpad and mouse mode #178). Letting the user switch the stream surface to relative trackpad movement gives full-screen landscape streaming the precision of the portrait trackpad without giving up screen space.

Checks

• Merged with main (52dbbde): the three conflicts kept both sides (main's zoom and key-repeat arguments to InputOverlay, then the Trackpad overlay).
• Tests/checks/tour: 732,453 checks, 129 of 129 mutants (6 checks and 3 mutants new); run-all.sh: all 63.
• Tests/checks/pointer-presence: 171 checks, 0 failed; 46 of 46 mutants caught (3 new).
• iOS Simulator Debug build (Sill.xcodeproj) clean.

What I tried

• On a 14-inch MacBook Pro (M4 Pro, macOS 27.0.1), Xcode 27.2: the three iOS builds (Debug and Release for the simulator, Debug for a device) and the pure checks above, after the merge with main and after each fix.
• Trackpad mode itself has not been run since the merge: not on a simulator, not on an iPad or iPhone. The scroll and hover fall-through and the zoom reset are checked by the build only.

Not tested

• The scroll and hover fall-through with a real Magic Keyboard trackpad or mouse (no harness sends scroll or hover events).