Implements discussion #599's phase 1 (Linux, libspnav): a 3Dconnexion
6-DOF device pans and zooms the active diagram view, via spacenavd.
## Off by default, zero cost when off
QET_ENABLE_SPACEMOUSE (cmake/developer_options.cmake) is OFF. Verified in
two separate build directories from a clean configure: with it off, the
new cmake/find_spacemouse.cmake step runs and does nothing (no library
lookup, no definition, no new source files compiled), and qetapp.cpp/.h
produce zero new object code -- both are entirely #ifdef'd out. The
default build is byte-for-byte the same shape as before this commit.
With it on, libspnav is located via its pkg-config file (spnav.pc, shipped
by libspnav-dev on Debian/Ubuntu and equivalent packages elsewhere). If the
option is on but the library isn't found, this does not hard-fail
configure: it downgrades back to off with a warning, so an opt-in feature
never blocks a developer who doesn't have the library installed.
## No new navigation logic -- a new input source for the existing one
DiagramView::wheelEvent() already turns a physical wheel's delta into
horizontalScrollBar()/verticalScrollBar() calls for pan and a
zoom(1 + value/1000) call for zoom -- see diagramview.cpp:661-687.
SpaceMouseListener calls exactly those same primitives from spnav motion
events instead of wheel events. It does not reimplement panning or
zooming.
## Safe by construction even when compiled in
The overwhelming majority of users of a build with the option on still
won't have spacenavd running or a device attached -- that must never
surface as an error dialog or a startup warning. SpaceMouseListener::
isAvailable() reflects this: spnav_open() failing is treated as the
ordinary case, not an error, and the object then does nothing at all.
Verified for real in this environment, which genuinely has no spacenavd
running: built with the option on, ran the resulting binary to completion,
and confirmed zero crashes and zero spnav-related output of any kind --
the silence is the point.
Motion is read via a QSocketNotifier on spnav_fd() (event-driven, no
polling loop, no idle cost) and applied to whichever DiagramView is
currently active, found via qApp->activeWindow() -> QETDiagramEditor ->
currentProjectView() -> currentDiagram(): a 6-DOF device is one ambient
input source for the whole application, not something tied to a
particular window, so there is exactly one listener, owned by QETApp.
## What could not be verified without hardware
The Z-axis-to-zoom-factor mapping (SpaceMouseListener::zoomFactorForZAxis)
is a pure function specifically so it could be tested without a live
device: confirmed a centered device (z=0) yields exactly 1.0 (an exact
no-op, not an epsilon-off value that could trip DiagramView::zoom()'s
>=1 branch), and that push/pull produce symmetric zoom-in/out factors.
What genuinely cannot be checked in this environment: which physical axis
is "left/right" vs "up/down" vs "push/forward", their sign, and whether
the ZOOM_DIVISOR/PAN_SCALE constants feel right on real hardware. Both are
named constants specifically so recalibrating them is a one-line change
once someone with a device tries it -- flagged plainly in the PR rather
than presented as verified.
## Not in this commit
Windows/macOS (proprietary 3DxWare SDK, materially bigger lift) and
device button mapping are both explicitly out of scope for this phase per
the discussion.
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