Aa: an Off detent, so 1× is not both "leave my window" and a fit (#179)

[ArfiArfath21]

Closes #179.

With no scale the host leaves the Mac window alone. The Aa control showed that state as 1×, the same as a fit at 1×, so once Aa had been touched nothing took a device back to "leave my window", and each selection shrank the window again until the person disconnected (the reporter's workaround).

What it does

• Aa's ruler gets an Off detent, one 0.25 step below 0.5×. An untouched Aa now shows Off (it used to read 1×), a tap on an untouched control still changes nothing, and dragging left of 0.5× lands on Off. VoiceOver says "Off", and decrement from 0.5× reaches it.
• Choosing Off sends no scale, as an untouched Aa always did.
• Host: when the connection that had a scale sends a viewport without one, the windows Sill sized go back to their own frames at once (restoreFits, the path a window takes when it stops streaming), instead of at the end of the session. It is tracked per connection (fitScales), so another device's viewport without a scale ends nobody's fit.
• With the virtual display on, no scale means 1× there, so the control does not offer Off (offered, from the Mac's setting as the device knows it).
• TextScale (new, Sources/StreamProtocol, so no project-file entry) holds the detents, snapping, titles and the "ends a fit" rule; the old TextScaleControl.snap and its detent list moved into it, and TextScaleControl.label still exists. 18 new checks in the protocol check, 8 mutants, all caught. Tests/checks/README.md and docs/DEVELOPMENT.md (Windows sized for a device) say so.

For review

• Decision for you: an untouched Aa now reads "Off" where it read "1×". It is what the issue asks for (the two states told apart), and I think it is the honest label, but it is a visible change, and a first drag right now needs three detents to reach 1×. A smaller alternative is to keep the untouched label and add Off only as a way back.
• StreamCoordinator.swift, the .viewport case: the restore happens before viewport is updated. Worth a look next to your fit-by-default branch and Could Sill scale apps while not using Virtual Display mode #70, which also touch when windows are fitted.
• The restore uses restoreFits(keeping: nil), which also puts back a window that is streaming now; the catalog's size check then restarts the pipeline, as for any resize.

Verified, and not

• Tests/checks/run-all.sh protocol passes (210 checks) and its eight new mutants are caught. swift build -c release --product SillHost builds. The iOS target type-checks with my Xcode 26.6 apart from the error main already has (DeviceHingeContext in DuoPostureReader.swift, which needs the 27.1 SDK).
• I have not run the app, and not on a live Mac: the ruler's look with a sixth detent, and the host putting a window back when Off is chosen, are untested end to end. The restore reuses the existing stop path, but I have not seen it fire.

AI assistance: this change was written by an AI coding assistant (Claude) at my direction. I reviewed the diff and ran the checks above.