Add capsules/turtle.4th, a limited LOGO-style turtle graphics demo capsule

First entry in the "cookbook" track: a demo capsule plus HOWTO, per the
sequencing laid out after the POST-coverage sweep. Built entirely in FORTH
on top of existing primitives -- fabric.4th's LINE (raster Bresenham) and
Q.SIN/Q.COS (Q48.16 trig), plus PLOT/FB-WIDTH/FB-HEIGHT -- no new C words.

FORWARD/BACK/LEFT/RIGHT/PENUP/PENDOWN/HOME/SETXY/SETHEADING/SETCOLOR give
the classic turtle model; POLYGON and STAR compose FORWARD+turn into simple
demo shapes; TURTLE-DEMO is a one-call visual smoke test. Not wired into
init.4th -- REPL-invoked only, matching the original idea's own scope.

Verified: mkcapsule --lint clean, hosted-build logic trace shows zero VM
errors and correct stack balance through the whole vocabulary, zero build
warnings and capsule loads cleanly on all three kernel architectures.
Visual pixel-level confirmation not yet done (needs an interactive
gtk-display session or driving past the ~25-30 min DoE-before-REPL wall),
documented as an open item in the HOWTO.

HOWTO: docs/working/architecture/TURTLE-GRAPHICS-HOWTO-20260819.md

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

capsules/turtle.4th

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+Block 5100
+( turtle.4th -- limited LOGO-style turtle graphics. Cookbook  )
+( demo capsule. Built on capsules/fabric.4th's LINE and       )
+( Q48.16 Q.SIN/Q.COS -- pure FORTH, no new C words. Load      )
+( fabric.4th first: uses LINE, FB-WIDTH, FB-HEIGHT, PLOT,     )
+( Q.SIN, Q.COS, Q.FROM-INT, Q.TO-INT, Q.*, Q.+.               )
+1144 CONSTANT DEG2RAD  ( Q48.16 rad/deg, pi/180 ~= 0.0174533 )
+VARIABLE TX  VARIABLE TY  VARIABLE THEAD
+VARIABLE TPEN  VARIABLE TCOLOR
+
+Block 5101
+( HOME: turtle to screen center, heading 0 (east), pen down, )
+( color white. Does not clear the screen -- see CS for that. )
+: HOME ( -- )
+  FB-WIDTH 2/ TX !
+  FB-HEIGHT 2/ TY !
+  0 THEAD !
+  1 TPEN !
+  16777215 TCOLOR ! ;
+
+Block 5102
+( PENUP / PENDOWN: whether FORWARD/BACK draw while moving.    )
+( SETCOLOR: 24-bit 0xRRGGBB, matches PLOT's own format.       )
+( SETXY: jump to (x y) without drawing, any pen state.        )
+( SETHEADING: absolute heading in degrees, 0=east, CCW+.      )
+: PENUP ( -- )      0 TPEN ! ;
+: PENDOWN ( -- )    1 TPEN ! ;
+: SETCOLOR ( color -- )  TCOLOR ! ;
+: SETXY ( x y -- )  TY ! TX ! ;
+: SETHEADING ( deg -- )  Q.FROM-INT DEG2RAD Q.* THEAD ! ;
+
+Block 5103
+( FORWARD / BACK: move n pixels along current heading        )
+( (THEAD, Q48.16 radians), drawing a LINE from old to new     )
+( position if pen is down. z is always 0 -- turtle graphics   )
+( is flat; fabric.4th's LINE just carries z through for the   )
+( 3D case it also supports.                                   )
+VARIABLE TNX  VARIABLE TNY
+: FORWARD ( n -- )
+  DUP Q.FROM-INT THEAD @ Q.COS Q.* Q.TO-INT TX @ + TNX !
+  Q.FROM-INT THEAD @ Q.SIN Q.* Q.TO-INT TY @ + TNY !
+  TPEN @ IF
+    TX @ TY @ 0 TNX @ TNY @ 0 TCOLOR @ LINE
+  THEN
+  TNX @ TX !  TNY @ TY ! ;
+: BACK ( n -- )  NEGATE FORWARD ;
+
+Block 5104
+( LEFT / RIGHT: turn in place by n degrees, CCW / CW.         )
+: LEFT ( deg -- )
+  Q.FROM-INT DEG2RAD Q.* THEAD @ Q.+ THEAD ! ;
+: RIGHT ( deg -- )  NEGATE LEFT ;
+
+Block 5105
+( CS: clear the framebuffer to black. framebuffer_words.c     )
+( only exposes PLOT (no fill primitive), so this is a plain   )
+( nested PLOT loop -- slow under TCG for a full screen, fine  )
+( for a one-shot demo clear, not meant to run per-frame.      )
+: CS ( -- )
+  FB-HEIGHT 0 DO
+    FB-WIDTH 0 DO
+      I J 0 PLOT
+    LOOP
+  LOOP ;
+
+Block 5106
+( POLYGON: regular polygon, SIDES corners, LEN pixels per     )
+( edge, drawn from the turtle's current position/heading      )
+( (call HOME first for a clean start). Classic LOGO idiom:     )
+( forward + turn 360/sides, repeated SIDES times.              )
+VARIABLE PLEN
+: POLYGON ( sides len -- )
+  PLEN !
+  DUP 0 DO
+    PLEN @ FORWARD
+    DUP 360 SWAP / LEFT
+  LOOP
+  DROP ;
+
+Block 5107
+( STAR: self-intersecting 5-pointed LOGO star -- forward +    )
+( a 144-degree turn (not 360/5=72, a plain pentagon), 5x.      )
+: STAR ( len -- )
+  DUP 5 0 DO
+    DUP FORWARD
+    144 LEFT
+  LOOP
+  DROP ;
+
+Block 5108
+( TURTLE-DEMO: visual smoke test -- clear screen, a cyan      )
+( hexagon, then a magenta star from the same start point.     )
+( Run after loading: S" turtle.4th" EXEC  TURTLE-DEMO          )
+: TURTLE-DEMO ( -- )
+  CS
+  HOME  65535 SETCOLOR     ( cyan  0x00FFFF )
+  6 80 POLYGON
+  HOME  16711935 SETCOLOR  ( magenta 0xFF00FF )
+  100 STAR ;