fix: replace std clamp with anchor regularization to prevent OOD noise

The std clamp was post-hoc and only addressed magnitude, not direction.
x0 was drifting to mean=-0.55/std=3.1 (ODE expected mean=0/std=1).

Replace with anchor_weight * MSE(x0, x0_init) added directly to the loss.
The optimizer now balances style matching against staying near the initial
N(0,1) noise — gradient-aware, prevents both magnitude and mean drift.

Also logs style/anchor losses and x0_std per step for diagnostics.

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

nodes/selva_ditto_optimizer.py

@@ -152,6 +152,13 @@ class SelvaDittoOptimizer:
                                "the mean spectrum loss. 0 = mean spectrum only (less noise). "
                                "0.1 adds texture matching but can introduce white noise.",
                 }),
+                "anchor_weight": ("FLOAT", {
+                    "default": 1.0, "min": 0.0, "max": 10.0, "step": 0.1,
+                    "tooltip": "L2 penalty keeping x0 near its initial N(0,1) noise. "
+                               "Prevents optimization from pushing x0 out of the flow's "
+                               "expected distribution (which causes white noise). "
+                               "Higher = cleaner audio, weaker style. 1.0 is a safe default.",
+                }),
                 "steps": ("INT", {
                     "default": 25, "min": 1, "max": 200,
                     "tooltip": "Euler steps for the final generation pass (after optimization).",
@@ -181,7 +188,7 @@ class SelvaDittoOptimizer:
 
     def optimize(self, model, features, prompt, negative_prompt,
                  reference_dir, n_opt_steps, opt_lr, n_ode_steps, n_grad_steps,
-                 style_weight, gram_weight, steps, cfg_strength, seed,
+                 style_weight, gram_weight, anchor_weight, steps, cfg_strength, seed,
                  normalize=True, target_lufs=-27.0):
         import traceback
 
@@ -286,7 +293,7 @@ class SelvaDittoOptimizer:
                     ref_mean, ref_gram,
                     seq_cfg, sample_rate, device, dtype,
                     n_opt_steps, opt_lr, n_ode_steps, n_grad_steps,
-                    style_weight, gram_weight, steps, cfg_strength, seed,
+                    style_weight, gram_weight, anchor_weight, steps, cfg_strength, seed,
                     normalize, target_lufs, pbar,
                 )
             except Exception as e:
@@ -312,7 +319,7 @@ def _do_optimize(net_generator, feature_utils, mel_converter,
                  ref_mean, ref_gram,
                  seq_cfg, sample_rate, device, dtype,
                  n_opt_steps, opt_lr, n_ode_steps, n_grad_steps,
-                 style_weight, gram_weight, steps, cfg_strength, seed,
+                 style_weight, gram_weight, anchor_weight, steps, cfg_strength, seed,
                  normalize, target_lufs, pbar):
     """Optimization loop — runs in a fresh thread (no inference_mode active)."""
 
@@ -384,7 +391,8 @@ def _do_optimize(net_generator, feature_utils, mel_converter,
         1, seq_cfg.latent_seq_len, net_generator.latent_dim,
         device=device, dtype=dtype,
     )
-    x0 = torch.nn.Parameter(x0_init.clone())
+    x0      = torch.nn.Parameter(x0_init.clone())
+    x0_init = x0_init.detach()          # anchor — kept fixed, no grad
     optimizer = torch.optim.Adam([x0], lr=opt_lr)
 
     # n_grad_steps must not exceed n_ode_steps
@@ -449,25 +457,25 @@ def _do_optimize(net_generator, feature_utils, mel_converter,
         # clips in the same space. Avoids backprop through VAE decoder which
         # is @torch.inference_mode() and produces noisy gradients.
         x_un = net_generator.unnormalize(x)          # [1, T_lat, C_lat]
-        loss = style_weight * _latent_style_loss(x_un.squeeze(0), ref_mean, ref_gram, gram_weight)
+        style_loss = style_weight * _latent_style_loss(x_un.squeeze(0), ref_mean, ref_gram, gram_weight)
+
+        # Anchor regularization — penalize x0 drifting from its initial N(0,1)
+        # value. Flow matching ODE expects x0 ~ N(0,1); large deviations push
+        # the ODE into an out-of-distribution region that decodes as white noise.
+        anchor_loss = anchor_weight * F.mse_loss(x0, x0_init)
+        loss = style_loss + anchor_loss
 
         optimizer.zero_grad()
         loss.backward()  # gradient flows through Phase 2 + STE back to x0.grad
         torch.nn.utils.clip_grad_norm_([x0], 1.0)
         optimizer.step()
 
-        # Clamp x0 std to stay near unit Gaussian — flow matching ODE expects
-        # x0 ~ N(0,1). Optimization can push std >> 1, which maps to an
-        # out-of-distribution initial condition and produces white noise.
-        with torch.no_grad():
-            std = x0.std()
-            if std > 1.5:
-                x0.data.div_(std)
-
         pbar.update(1)
 
         if (opt_step + 1) % max(1, n_opt_steps // 10) == 0:
-            print(f"[DITTO] {opt_step+1}/{n_opt_steps}  loss={loss.item():.4f}", flush=True)
+            print(f"[DITTO] {opt_step+1}/{n_opt_steps}  "
+                  f"style={style_loss.item():.4f}  anchor={anchor_loss.item():.4f}  "
+                  f"x0_std={x0.data.std().item():.3f}", flush=True)
 
     # ── Final generation with optimized x_0 ─────────────────────────────────
     print(f"[DITTO] Optimization done. Final generation ({steps} steps)...", flush=True)