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

hw/sim/sweep_noise.py

@@ -22,17 +22,20 @@ def apply_write_noise(
     width: int,
     rate_num: int,
     rate_den: int,
-    noise_gen_bits: int = 8,
-    noise_sample_bits: int = 8,
+    noise_bits: int = 8,
     seed: int = 0,
 ) -> list[int]:
-    """Apply write-noise flip masks to every row, returning noisy copies."""
+    """Apply write-noise flip masks to every row, returning noisy copies.
+
+    *seed* is a 64-bit value (RTL NOISE_SEED). It is duplicated to form
+    the 128-bit xorshift initial state: {seed, seed}.
+    """
     noisy: list[int] = []
-    state = seed
+    state = (seed << 64) | seed
     mask_w = (1 << width) - 1
     for row in rows:
         flip, state = generate_write_flip_mask(
-            state, width, noise_gen_bits, noise_sample_bits, rate_num, rate_den
+            state, width, noise_bits, rate_num, rate_den
         )
         noisy.append((row ^ flip) & mask_w)
     return noisy
@@ -44,8 +47,7 @@ def main() -> None:
     parser.add_argument("--queries", type=int, default=128)
     parser.add_argument("--width", type=int, default=512)
     parser.add_argument("--seed", type=int, default=1234)
-    parser.add_argument("--noise-gen-bits", type=int, default=8)
-    parser.add_argument("--noise-sample-bits", type=int, default=8)
+    parser.add_argument("--noise-bits", type=int, default=8)
     parser.add_argument(
         "--rates",
         type=float,
@@ -80,8 +82,7 @@ def main() -> None:
             width=args.width,
             rate_num=rate_num,
             rate_den=rate_den,
-            noise_gen_bits=args.noise_gen_bits,
-            noise_sample_bits=args.noise_sample_bits,
+            noise_bits=args.noise_bits,
             seed=args.seed,
         )