docs: v1.5.1 fix — factored legend arithmetic correction (P1)

§1.5 of dfdraw_Technical_Summary.md and the equivalent claim in
PHASE_HISTORY.md both stated:
  '|g|=5, |v|=3, |q|=5 produces 11 entries factored vs 75 unfactored'

Sum of cardinalities is 5+3+5 = 13, not 11. Product (75) was correct.
Caught by Sonet50 + Sonet51 in the Technical Summary review cycle by
running the math through Python — Claude40 and GPT1 both missed it.
Source error appears to originate at channels.py:291 docstring (uses
cardinalities (5, 3, 3) → sum 11 ✓, but product 45 ≠ 75 stated).

Two mechanical 11→13 changes; no semantic changes.

Approval: Claude40 [OK] + GPT1 [!] + Sonet50 [!] = 3-reviewer threshold
met after this fix per consolidator recommendation.

Author: miranov25