feat(cam): migrate noise generation from xorshift64 to xorshift128

- Replace NOISE_GEN_BITS/NOISE_SAMPLE_BITS parameters with unified NOISE_BITS
- Use xorshift128 (random128) instead of xorshift64 for PRNG
- Add flip_mask_next combinational helper for single-cycle mask computation
- Add random_enable signal to advance PRNG only on accepted noisy writes
- Simplify FSM by removing mask_group_idx counter
- Update parameter validation: GROUP_BITS (= HASH_BITS/NOISE_BITS) must equal 64
- Update ref_model.py and tests to match new seed convention: {seed, seed}
- Update Makefile and sweep_noise.py with renamed parameters

.opencode/opencode.json

@@ -6,7 +6,9 @@
       "python *": "deny",
       "make": "deny",
       "make *": "deny",
-      "just remote *": "ask"
+      "just remote *": "ask",
+      "verilator": "deny",
+      "verilator *": "deny"
     },
     "read": {
       ".env": "deny"