fabric: em-square glyph coordinate convention (EM-X/EM-Y/G-LINE)

Punch list §25 item 4.3.6 complete.
capsules/fabric.4th blocks 4916-4917: EM-UNITS, GOX/GOY/GSIZE/GCOLOR,
EM-X/EM-Y/G-LINE per §27.6.1, scaling/translating em-square strokes
into CART-PLOT screen coordinates via */. Verified live on amd64 via
a temporary probe (built, run once, reverted): two G-LINE test shapes
at GSIZE 100 and GSIZE 50, one leg each exercising a negative em-y
value chosen to hit */'s truncate-toward-zero behaviour, not a
multiple of EM-UNITS. Screendump pixel-bbox extraction matched
hand-calculated raster coordinates exactly on both shapes. Three-arch
acceptance boot clean, Stadium conservation unaffected
(resident_sum=43691 reservoir=21845 sum=65536 on all three).

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

capsules/fabric.4th

@@ -177,3 +177,23 @@ Block 4915
   7 6 CCOLOR @ EDGE  6 4 CCOLOR @ EDGE
   0 4 CCOLOR @ EDGE  1 5 CCOLOR @ EDGE
   2 6 CCOLOR @ EDGE  3 7 CCOLOR @ EDGE ;
+
+Block 4916
+( EM-X/EM-Y: em-square (1000 units/em) -> CART-PLOT space.  )
+( Item 4.3.6. GOX/GOY are the glyph's baseline-left anchor, )
+( GSIZE the requested pixel size -- set by DRAW-GLYPH later )
+( (item 4.3.6b), not yet built.                              )
+1000 CONSTANT EM-UNITS
+VARIABLE GOX  VARIABLE GOY  VARIABLE GSIZE  VARIABLE GCOLOR
+: EM-X ( em-x -- cart-x )  GSIZE @ EM-UNITS */ GOX @ + ;
+: EM-Y ( em-y -- cart-y )  GSIZE @ EM-UNITS */ GOY @ + ;
+
+Block 4917
+( G-LINE: one glyph stroke, em-square coords -> CART-PLOT.  )
+VARIABLE GX1 VARIABLE GY1 VARIABLE GX2 VARIABLE GY2
+: G-LINE ( gx1 gy1 gx2 gy2 -- )
+  GY2 ! GX2 ! GY1 ! GX1 !
+  GX1 @ EM-X  GY1 @ EM-Y  0
+  GX2 @ EM-X  GY2 @ EM-Y  0
+  GCOLOR @
+  LINE ;