Add subframe geometry keywords and run exposures on a worker thread

[mikelangmayr]

• Add the tracking camera's subframe keywords: subframemode, y0/y1/x0/x1, subframeheight/subframewidth and geometrystatus. Bounds and sizes read in either mode; each is writable only in the mode that accepts it, and a rejected write names the keyword that does work there
• Apply the camera mode, readout mode, window and ROI as one sequence, with the camera mode first since it reloads its parameters from the ACF
• Run expose on a worker thread so it returns at once and progress shows in state; a second expose is refused while one runs
• Make abort interrupt a run and settle to idle rather than error, and report the written path in lastframe once the asynchronous writer has flushed
• Verified against the Archon emulator: a 64x64 centred ROI reports bounds centred on the detector, per-mode write rejections behave as intended, and an aborted run settles to idle. pylint 10.00/10