Add FITS header dictionary and wire it into the async writer

[mikelangmayr]

• FrameMetadata carries two generic Common::FitsKeys blobs (header_set, frame_keys); FitsWriter emits both with no instrument-specific keyword knowledge
• HISPEC's property dictionary lives in the instrument submodule instead (companion PR: hispec-tracking-camera-instrument#11)
• key command, lifted from v1: adds custom FITS keywords via userkeys, no writer-side changes needed
• datacube command, lifted from v1: writes one multi-extension FITS file per exposure (one image extension per read) instead of one file per read; each extension carries its own frame_keys plus a new WRTTIME
• FrameOutput::end_exposure() finalizes the current cube when the exposure command ends (not by extension count); dropped the unused CadenceGate frame-rate throttle
• Verified on real hardware on fei: headers, key, and datacube (including two separate exposures producing two separate cube files) all confirmed correct