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_sim.py

@@ -197,17 +197,21 @@ def sat_sub_q(a, b):
     return sat_add_q(a, -b)
 
 
-def min_sum_cn_update(msgs_in, offset=OFFSET):
+def min_sum_cn_update(msgs_in, offset=OFFSET, cn_mode='offset', alpha=0.75):
     """
-    Offset min-sum check node update.
+    Min-sum check node update with offset or normalized mode.
 
     For each output j:
       sign = XOR of all other input signs
-      magnitude = min of all other magnitudes - offset (clamp to 0)
+      magnitude = min of all other magnitudes, corrected by:
+        - offset mode: mag = max(0, mag - offset)
+        - normalized mode: mag = floor(mag * alpha)
 
     Args:
         msgs_in: list of DC signed integers (Q-bit)
-        offset: offset correction value
+        offset: offset correction value (used in offset mode)
+        cn_mode: 'offset' or 'normalized'
+        alpha: scaling factor for normalized mode (default 0.75)
 
     Returns:
         msgs_out: list of DC signed integers (Q-bit)
@@ -232,7 +236,10 @@ def min_sum_cn_update(msgs_in, offset=OFFSET):
     msgs_out = []
     for j in range(dc):
         mag = min2 if j == min1_idx else min1
-        mag = max(0, mag - offset)  # offset correction
+        if cn_mode == 'normalized':
+            mag = int(mag * alpha)
+        else:
+            mag = max(0, mag - offset)
         sgn = sign_xor ^ signs[j]   # extrinsic sign
         val = -mag if sgn else mag
         msgs_out.append(val)