feat: add LDPC demo firmware and cocotb test

Demo firmware runs clean decode + noisy decode (vector 0) and reports
pass/fail on GPIO[7:0]. All 5 cocotb tests pass: ldpc_basic, ldpc_noisy,
ldpc_max_iter, ldpc_back_to_back, and ldpc_demo.

Also adds .cf/project.json with GPIO configuration.

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

.cf/project.json

@@ -0,0 +1,49 @@
+{
+  "project_name": "ldpc_decoder_optical",
+  "description": "LDPC decoder for photon-starved optical communication",
+  "project_type": "digital",
+  "pdk": "sky130A",
+  "created": "2026-02-25",
+  "project": {
+    "gpio_config": {
+      "0": "13'h1809",
+      "1": "13'h1809",
+      "2": "13'h1809",
+      "3": "13'h1809",
+      "4": "13'h1809",
+      "5": "13'h1809",
+      "6": "13'h1809",
+      "7": "13'h1809",
+      "8": "13'h1809",
+      "9": "13'h1809",
+      "10": "13'h1809",
+      "11": "13'h1809",
+      "12": "13'h1809",
+      "13": "13'h1809",
+      "14": "13'h1809",
+      "15": "13'h1809",
+      "16": "13'h1809",
+      "17": "13'h1809",
+      "18": "13'h1809",
+      "19": "13'h1809",
+      "20": "13'h1809",
+      "21": "13'h1809",
+      "22": "13'h1809",
+      "23": "13'h1809",
+      "24": "13'h1809",
+      "25": "13'h1809",
+      "26": "13'h1809",
+      "27": "13'h1809",
+      "28": "13'h1809",
+      "29": "13'h1809",
+      "30": "13'h1809",
+      "31": "13'h1809",
+      "32": "13'h1809",
+      "33": "13'h1809",
+      "34": "13'h1809",
+      "35": "13'h1809",
+      "36": "13'h1809",
+      "37": "13'h1809"
+    }
+  }
+}