fabric: UTF-8 decoder (DECODE-UTF8)

Punch list §25 item 4.3.6a complete.
capsules/fabric.4th blocks 4918-4920: UTF8-SEQ-LEN/UTF8-CONT? verbatim
from §27.6.2, DECODE-UTF8 dispatching to four UTF8-ASSEMBLE-N helpers
(1/2/3/4-byte). Factored into helpers after the single-word draft hit
mkcapsule's real 64-char x 16-line block limit. Verified live on
amd64: a 6-byte test buffer (ASCII + deg-sign + em-dash) decoded to
65/176/8212 across three sequential calls, exact match against
hand-computed codepoints, buffer fully consumed. Three-arch acceptance
boot clean, Stadium conservation unaffected.

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

capsules/fabric.4th

@@ -197,3 +197,45 @@ VARIABLE GX1 VARIABLE GY1 VARIABLE GX2 VARIABLE GY2
   GX2 @ EM-X  GY2 @ EM-Y  0
   GCOLOR @
   LINE ;
+
+Block 4918
+( UTF-8 lead-byte classification. Item 4.3.6a.               )
+: UTF8-SEQ-LEN ( lead -- n )      ( 0 = invalid lead byte )
+  DUP 128 < IF DROP 1 EXIT THEN
+  DUP 224 AND 192 = IF DROP 2 EXIT THEN
+  DUP 240 AND 224 = IF DROP 3 EXIT THEN
+  DUP 248 AND 240 = IF DROP 4 EXIT THEN
+  DROP 0 ;
+: UTF8-CONT? ( byte -- flag )     ( true if 10xxxxxx )
+  192 AND 128 = ;
+
+Block 4919
+( UTF-8 codepoint assembly, per byte-length. Item 4.3.6a.   )
+VARIABLE UADDR VARIABLE ULEN VARIABLE ULEAD
+VARIABLE USEQLEN VARIABLE UCP
+: UTF8-ASSEMBLE-1 ( -- cp )  ULEAD @ ;
+: UTF8-ASSEMBLE-2 ( -- cp )
+  ULEAD @ 31 AND 6 LSHIFT
+  UADDR @ 1+ C@ 63 AND OR ;
+: UTF8-ASSEMBLE-3 ( -- cp )
+  ULEAD @ 15 AND 12 LSHIFT
+  UADDR @ 1+ C@ 63 AND 6 LSHIFT OR
+  UADDR @ 2 + C@ 63 AND OR ;
+
+Block 4920
+( UTF8-ASSEMBLE-4 + DECODE-UTF8 dispatch. Item 4.3.6a.      )
+: UTF8-ASSEMBLE-4 ( -- cp )
+  ULEAD @ 7 AND 18 LSHIFT
+  UADDR @ 1+ C@ 63 AND 12 LSHIFT OR
+  UADDR @ 2 + C@ 63 AND 6 LSHIFT OR
+  UADDR @ 3 + C@ 63 AND OR ;
+: DECODE-UTF8 ( addr u -- codepoint addr' u' )
+  ULEN ! UADDR !
+  UADDR @ C@ ULEAD !
+  ULEAD @ UTF8-SEQ-LEN USEQLEN !
+  USEQLEN @ 1 = IF UTF8-ASSEMBLE-1 UCP ! ELSE
+  USEQLEN @ 2 = IF UTF8-ASSEMBLE-2 UCP ! ELSE
+  USEQLEN @ 3 = IF UTF8-ASSEMBLE-3 UCP ! ELSE
+  USEQLEN @ 4 = IF UTF8-ASSEMBLE-4 UCP ! ELSE
+  65533 UCP ! 1 USEQLEN ! THEN THEN THEN THEN
+  UCP @ UADDR @ USEQLEN @ + ULEN @ USEQLEN @ - ;