Software: pattern player, web page with a design library, GRBL tools

- python3 -m sandtable web: local web page to play designs from a library of
  27 built-in patterns, upload .thr or .svg, pause/skip/stop, jog and calibrate.
  Reconnects by itself when the USB cable comes back.
- Plays .thr files and built-in generators with an erase spiral between them,
  constant ball speed, host-side homing on stock GRBL 1.1 (Uno + CNC shield),
  FluidNC for the ESP32 board.
- Home switch guard: patterns never press the switch, so a press means lost
  steps and stops the machine at once with an alert.
- lineart.py turns SVG line drawings into one continuous path.
- Tests against a GRBL simulator.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

software/sandtable.toml

@@ -0,0 +1,27 @@
+# Sand table settings. Anything left out uses the default in sandtable/config.py.
+
+[serial]
+port = "auto"          # or e.g. "/dev/ttyACM0" (Uno) / "/dev/ttyUSB0" (CH340 clones)
+baud = 115200
+
+[controller]
+type = "grbl"          # "grbl" = Arduino Uno + CNC shield; "fluidnc" = the custom ESP32 board
+
+[machine]
+rho_max_mm = 133.0     # how far out the ball goes (the tray reaches 133 mm)
+invert_theta = false   # set to true if test_f draws a mirrored F
+theta_offset_deg = 0.0 # turn every pattern by this much
+
+# CNC shield DRV8825s at 1/32 (M0, M1 and M2 jumpers all fitted, 2026-10-31).
+# Only M2 fitted (1/16) measured as 35.556 / 80.
+theta_steps_per_deg = 71.111
+rho_steps_per_mm = 160.0
+
+[motion]
+ball_speed_mm_s = 25.0 # drawing speed; slower = crisper lines and quieter
+max_theta_deg_s = 60.0 # arm spin limit (10 rpm)
+
+[erase]
+enabled = true
+pitch_mm = 7.0         # spiral spacing between patterns (the ball is 9.5 mm)
+speed_mm_s = 35.0