data: add SC-LDPC results and comprehensive comparison plots

SC-LDPC threshold saturation results:
- SC original staircase: 2.28 photons/slot (vs 4.76 uncoupled)
- SC DE-optimized: 1.03 photons/slot (vs 3.21 uncoupled)
- Shannon limit: 0.47 photons/slot (remaining gap: 3.4 dB)

Add CLI to sc_ldpc.py (threshold, fer-compare, full subcommands).
Add threshold progression, SC threshold bars, and SC FER plots.

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

data/sc_ldpc_results.json

@@ -0,0 +1,60 @@
+{
+  "uncoupled_thresholds": {
+    "original_offset": 4.7640625,
+    "optimized_offset": 3.2093749999999996,
+    "optimized_normalized": 3.2093749999999996
+  },
+  "sc_thresholds": {
+    "sc_original": 2.2765625,
+    "sc_optimized": 1.0328125
+  },
+  "shannon_limit": 0.47,
+  "params": {
+    "L": 20,
+    "w": 2,
+    "z_pop": 5000,
+    "tol": 0.5
+  },
+  "fer_comparison": {
+    "lam_s_points": [
+      2.0,
+      3.0,
+      4.0,
+      5.0,
+      7.0,
+      10.0
+    ],
+    "sc_fer": {
+      "2.0": {
+        "fer": 1.0,
+        "ber": 0.010056818181818182
+      },
+      "3.0": {
+        "fer": 1.0,
+        "ber": 0.006491477272727273
+      },
+      "4.0": {
+        "fer": 0.68,
+        "ber": 0.0003693181818181818
+      },
+      "5.0": {
+        "fer": 0.66,
+        "ber": 0.00037642045454545453
+      },
+      "7.0": {
+        "fer": 0.48,
+        "ber": 0.00022017045454545455
+      },
+      "10.0": {
+        "fer": 0.0,
+        "ber": 0.0
+      }
+    },
+    "params": {
+      "L": 10,
+      "w": 2,
+      "z": 32,
+      "n_frames": 50
+    }
+  }
+}