feat: add normalized min-sum CN update mode

Add cn_mode ('offset'/'normalized') and alpha parameters to
min_sum_cn_update() in ldpc_sim.py and generic_decode() in
ldpc_analysis.py. Normalized mode scales magnitudes by alpha
(default 0.75) instead of subtracting a fixed offset, which
is better suited for low-rate codes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

model/ldpc_analysis.py

@@ -152,7 +152,8 @@ def ira_encode(info, H_base, H_full, z=32):
     return codeword
 
 
-def generic_decode(llr_q, H_base, z=32, max_iter=30, early_term=True, q_bits=6):
+def generic_decode(llr_q, H_base, z=32, max_iter=30, early_term=True, q_bits=6,
+                   cn_mode='offset', alpha=0.75):
     """
     Parameterized layered min-sum decoder for any QC-LDPC base matrix.
 
@@ -166,6 +167,8 @@ def generic_decode(llr_q, H_base, z=32, max_iter=30, early_term=True, q_bits=6):
         max_iter: maximum iterations
         early_term: stop when syndrome is zero
         q_bits: quantization bits
+        cn_mode: 'offset' or 'normalized'
+        alpha: scaling factor for normalized mode (default 0.75)
 
     Returns:
         (decoded_info_bits, converged, iterations, syndrome_weight)
@@ -204,7 +207,10 @@ def generic_decode(llr_q, H_base, z=32, max_iter=30, early_term=True, q_bits=6):
         msgs_out = []
         for j in range(dc):
             mag = min2 if j == min1_idx else min1
-            mag = max(0, mag - offset)
+            if cn_mode == 'normalized':
+                mag = int(mag * alpha)
+            else:
+                mag = max(0, mag - offset)
             sgn = sign_xor ^ signs[j]
             val = -mag if sgn else mag
             msgs_out.append(val)