book_layout: wider per-character spacing so multi-char labels don't collide

Labels like 'logits'/'target'/'exp' were modeled ~15% too narrow
(CHAR_W=0.11), so adjacent multi-character nodes overlapped into an
illegible run-on ('logitstargetpow') and the short connecting wires
shrank to dots. Bumped CHAR_W to 0.14, matching the rendered roman
width, so those labels now sit apart with a visible wire between them.
Single-character labels (A, x, v) are unchanged. 38 tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018PzG3QbNtaFmABBHBG39wp

Author: thomasahle

tensorgrad/extras/book_layout.py

@@ -91,7 +91,7 @@
 # ---------------------------------------------------------------------------
 
 PITCH = 0.62  # horizontal distance between adjacent spine node centers
-CHAR_W = 0.11  # extra pitch per label character beyond the first
+CHAR_W = 0.14  # extra pitch per label character beyond the first
 STUB = 0.32  # length of a free-edge stub
 PENDANT_DY = 0.55  # vertical drop of a hanging branch level
 MIDSTUB = 0.4  # length of an interior free-edge stub (drawn upward)