[addhumal]

Summary

Towards #178 (@sirasterthecat-png: "an optional direct-touch drag gesture … while preserving the current scrolling/right-click behavior"). Device only, no wire change.

A finger held on the picture for 0.45 s:

• lifted where it was pressed (within 8 pt): a right click there, as before, but at the lift instead of at 0.45 s;
• moved on past 8 pt: the left button goes down where it was pressed, follows the finger, and comes up where it lifts, so a window, a slider or a selection moves under the finger.

A tap still clicks and a finger that moves before the hold still scrolls. A cancel or a three-finger stroke mid-drag lets the button up and clicks nothing. The drag is the left button's down, moves and up, as the Pencil's already are, so every Sill for Mac takes it.

• TouchHold (iOSClient/TouchHold.swift, pure): the rule. InputOverlayView's long press feeds it and sends what it returns.
• The tour's touch row: "Touch and hold to right-click, or hold, then drag, to move a window or select text."
• Tests/touchrig: RIG_ONLY=hold runs the real overlay's holds; a stand-in for ClipboardPolicy.pasteKey so the rig builds again on main; H7's compare.py holds only the trackpad's hold-and-move to the events before.

Checks

• Tests/checks/touch-hold (new, in CI's mutants matrix): 32 checks (20,000 random holds against a model: no button left down), 21 of 21 mutants caught.
• Tests/checks/tour: the new row's words; run-all.sh: all 64.
• The iOS app: Debug and Release for the simulator, Debug for a device; only the known StreamClient warning.
• The touch rig on a simulator, the real overlay with synthesized touches. Main: the hold-and-move right-clicked at 0.47 s and dropped the moves. This branch: pointer to the finger at 0.45 s, left down once past 8 pt, each move, left up at the lift. The tap, the one-finger scroll and its coast: the same events as main.

What I tried

• On a 14-inch MacBook Pro (M4 Pro, macOS 27.0.1), Xcode 27.2: the touch rig (RIG_ONLY=hold Tests/touchrig/run.sh), the real InputOverlayView driven by synthesized touches on an iPad Pro 11-inch (M5) simulator, iOS 27.0, against main's overlay (--rev upstream/main). A hold lifted in place right-clicked at the lift (634 ms; main 467 ms), a hold moved 60 pt sent left down at the start, ten moves and left up at the lift (main: a right click, the moves dropped), a cancel mid-drag let the button up, and a tap and a one-finger scroll were main's events exactly.
• No real iPad or iPhone yet, and no Mac receiving the drag from a device (the host side is the Pencil's, unchanged).

Not tested

• A real finger on an iPad or iPhone (the rig's touches are synthesized): the feel of the 8 pt slop, and a right click that now comes at the lift.
• A second finger landing mid-drag (on the rig: the picture zooms, the Mac's pointer holds still, the button comes up at the lift).
• With Option to use the stream picture as a relative trackpad (#178) #222 (Trackpad mode): whichever lands second keeps both tour rows' words.