Fix DRAW M/S/A bugs, add TA rotation, =variable; substitution, X substring execution

Rewrites gfx_draw() as a recursive draw_engine() to support all DRAW
mini-language features:

Bug fixes:
- M command parsing: skip generic arg parser so M100,50 correctly
  parses both coordinates instead of consuming x as a generic arg
- S (scale) semantics: distance is now (arg ?: 1) * scale / 4, matching
  original GW-BASIC where S4 means 1 pixel per unit, not 4
- A (rotation): implements 90-degree rotation state with direction
  vector transform for all 8 direction commands

New features:
- TA n: arbitrary rotation angle (-360 to 360 degrees) via cos/sin
- =variable;: numeric variable substitution in DRAW strings
- X stringvar;: execute substring from string variable (recursive)
- Scale factor applied to relative M coordinates

tests/programs/draw_commands.bas

@@ -0,0 +1,43 @@
+10 REM DRAW command comprehensive test
+20 SCREEN 2
+30 REM --- Bug 1: M command parsing (absolute) ---
+40 DRAW "M100,50"
+50 PRINT "M abs:"; POINT(100,50)
+60 REM --- Bug 1: M command parsing (relative) ---
+70 DRAW "BM10,10 M+20,+10"
+80 PRINT "M rel:"; POINT(30,20)
+90 REM --- Bug 2: S scale semantics ---
+100 REM Default S4: U with no arg moves 1 pixel
+110 DRAW "BM50,50 U"
+120 PRINT "S4 U:"; POINT(50,49)
+130 REM S8: U with no arg moves 2 pixels
+140 DRAW "BM50,50 S8 U"
+150 PRINT "S8 U:"; POINT(50,48)
+160 REM S8: U5 moves 10 pixels (5*8/4=10)
+170 DRAW "BM50,50 S8 U5"
+180 PRINT "S8 U5:"; POINT(50,40)
+190 REM --- Bug 3: A rotation ---
+200 DRAW "BM80,80 A1 R10"
+210 PRINT "A1 R:"; POINT(80,70)
+220 DRAW "A2 BM80,80 R10"
+230 PRINT "A2 R:"; POINT(70,80)
+240 DRAW "A0"
+250 REM --- TA arbitrary rotation ---
+260 DRAW "BM60,60 TA90 R10"
+270 PRINT "TA90 R:"; POINT(60,50)
+280 DRAW "TA0"
+290 REM --- =variable; substitution ---
+300 D%=20
+310 DRAW "BM10,150 R=D%;"
+320 PRINT "=var:"; POINT(15,150)
+330 REM --- X substring execution ---
+340 A$="U10R10D10L10"
+350 DRAW "BM120,120"
+360 DRAW "XA$;"
+370 PRINT "X sub:"; POINT(120,110)
+380 PRINT "X sub:"; POINT(130,120)
+390 REM --- Scale on relative M ---
+400 DRAW "S8 BM50,100 M+10,+0"
+410 PRINT "S M+:"; POINT(70,100)
+420 DRAW "S4"
+430 PRINT "DONE"