developer_panel: Add Missing button appends a tab per input attribute

Next to Rebuild Visualizers. It reads the input's point attributes, skips
P, N and Cd and any attribute no visualizer suits (strings, matrices,
vector2/4), appends a tab for each one no tab names yet -- Marker for a
float vector, Color for a float or int scalar -- and rebuilds. Only the
panel's input is seen, so an attribute created further down the chain
still gets its tab by hand.

It is a separate button so that Rebuild stays idempotent: a tab removed
on purpose must not come back on the next rebuild.

Verified headlessly (tab logic) and in a shadow Houdini 22.0.429 session
(button press creates matching vis_color / vis_marker visualizers).

Co-Authored-By: Claude Fable 5.1 <[email protected]>

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