Kiri:Moto todo

`C` cosmetic, `F` functional, `P` performance, `B` bug fix

B fix adaptive slicing with multi-extruder
B fix supports with grouped parts
F control for size of purge block (with 0=disabled)
F add separate fill speed control
F manual support addition / control
F polishing and other non-planar work
F gradient infill https://www.youtube.com/watch?v=hq53gsYREHU&feature=emb_logo
F feather sharp tips by reducing extrusion in area of overlap
F first layer segment large flat areas for better fill reliability
F enable purge block when quick layers are detected
F apply finish speed to exposed top and underside flat areas
F expand internal supporting flats
F first layer support speed should be same as shell speed
F determine start point from gcode preamble
F trim support offset from layer below
F option to support interior bridges when 0% infill
F calculate filament use per extruder per print
P refactor thin fill to use outline and inside poly normal dist to self
P segment large polygons for extremely large parts / infill
P implement infill clipping in wasm

P prioritize supports by length of unsupported span. mandatory when circularity > X

or % area of inner to outer poly is high (making it a thin shell)

B fails in pancaking (clone) when there are no sliced layers (like z bottom too high)
B contouring should extend beyond part boundaries by tool radius
B outside cutting direction in roughing mode inverted
F z bounded slices (extension of z bottom offset feature)
F use arcs to connect hard angles
F lead-in milling
F trapezoidal tabs (in the Z axis)
F ease-in and ease-out especially on tab cut-out start/stop
F implement z line-only follows for ball/taper
F add option to spiral in vs out (optimal tool life) vs mixed (optimal path)
F add endmill spiral direction to fully respect climb vs conventional
F add support for tapered ball mills
F warn when part > stock or cuts go outside bed
F add M03 tool feedrate support
P refactor slicing around flats w/ interpolation instead of culling
P disable topo generation when no contour xy and no depth first
P store tab and camshell polys in widget.topo to minimize z on edge moves
P contouring is going back to z top too often
P option to skip milling holes that would be drilled
P crossing open space check point is outside camshell before returning max z
P background worker to speculatively generate topo maps (and maybe pre-slicing)