WIP. Add stereonet module for plotting structural geology stereonets (Schmidt/Wulff)

[Esteban82]

Opening this as a draft/WIP specifically to get feedback before going further. @rhum1-geo, your suggestions (density contouring, Fisher stats, notation auto-detection, richer -S/vector options) are noted and intentionally not in this first PR — the plan is to land plotting first, then follow up with statistics in a later PR once the plotting side is settled.

What's implemented:

-T[d|l|p] — planes as strike/dip (right-hand rule) or dip-direction/dip, or lines as trend/plunge
-W/-S/-L/-G — cyclographic traces and poles (or lines), styled like plot
-A/-B — azimuth ring and the net's own grid mesh, both opt-in
-T+u — upper hemisphere

Tested with (no automated tests yet — these are the scripts I manually ran and checked; anyone from #9156 is welcome to run them too):

0_Empty_Nets.sh — empty Schmidt and Wulff nets, with title, grid, and azimuth ring, no data

gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray

gmt begin "0_Empty_Net_Schdmit_(A)" png
  echo /dev/null | gmt stereonet -JA10c -B+t"Schdmit" -B -A
gmt end

gmt begin "0_Empty_Net_Wullf_(S)" png
   echo /dev/null | gmt stereonet -JS10c -B+t"Wullf" -B -A
gmt end

1_Plot_Data.sh — a plane + its pole from strike/dip (RHR) input, styled independently

# RHR. Rhumb Dip
cat << EOF > data.txt
270 30
180 10
EOF

gmt begin "1_Plot_Data" png
  gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray
  gmt stereonet data.txt -JS10c -Sc0.5c -Gorange -L1p,blue -l"Pole" -B -A
  gmt stereonet data.txt -JS10c -W1p,red -l"Plane" 
gmt end
rm -f data.txt gmt.*

2_Plot_Data_Inverted.sh — same data with columns swapped; -: and -i1,0 should reproduce script 1 exactly

cat << EOF > data.txt
# Dip Rhumb
30 270
10 180
EOF

gmt begin "2_Plot_Data_Inverted" png
  gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray
  gmt stereonet data.txt -: -JS10c -Sc0.5c -Gorange -L1p,blue -l"Pole" -B -A
  gmt stereonet data.txt -i1,0 -JS10c -W1p,red -l"Plane"
gmt end
rm -f data.txt gmt.*

3_Plot_Planes_Conventions.sh — the same plane entered as -Tp (strike/dip, RHR) and -Td (dip-direction/dip); traces should coincide

cat << EOF > data1.txt
# Rhumb Dip
270 30
180 10
EOF

# Dip-Direction Dip
cat << EOF > data2.txt
0 30
270 10
EOF

gmt begin 3_Plot_Planes_Conventions png
	gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray
	gmt stereonet data1.txt -JA12c -W2p,red    -Sc0.4c -Gred    -Tp -l"-Tp: RHR" -B -A
	gmt stereonet data2.txt        -W1p,yellow -Sc0.2c -Gyellow -Td -l"-Td: Dip direction/Dip"
gmt end
rm data*.txt

4_Plot_Poles_Conventions.sh — -Tp and -Td poles for the same two planes should coincide (red under blue); -Tl reuses the same numbers as a line's trend/plunge instead, which is a different quantity and lands elsewhere on purpose

# RHR: Rhumb Dip
cat << EOF > data1.txt
270 30
180 10
EOF

# Dip-Direction Dip
cat << EOF > data2.txt
0 30
270 10
EOF

# Trend / Plunge
cat << EOF > data3.txt
0 30
270 10
EOF

gmt begin 4_Plot_Poles_Conventions png
	gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray
	gmt stereonet data1.txt -Sc0.4c -Gred   -Tp -l"-Tp: RHR" -B
	gmt stereonet data2.txt -Sc0.2c -Gblue  -Td -l"-Td: Dip direction/Dip"
	gmt stereonet data3.txt -Sc0.2c -Ggreen -Tl -l"Lines: Trend/Plunge"   
gmt end
rm data*.txt

5_Annotations.sh — fractional azimuth interval (-A22.5) with a title and -Vi

gmt set MAP_GRID_PEN_PRIMARY 0.25p,gray
gmt begin "5_Annotations" png
  echo /dev/null | gmt stereonet -JA10c -B+t"Schdmit" -A22.5 -Vi -B
gmt end

Model/effort: Claude Opus 5 for design and code review, Claude Sonnet 5 for iteration and testing.

Related to #9156.