Fix FlashVSR quality: two-stage temporal padding, kv_ratio=3, float64 precision

Root cause of remaining ghosting: our single-stage temporal padding
(N+4 → floor to 8k+1) TRUNCATED frames when N+4 wasn't already 8k+1.
For 50 frames: 50+4=54 → floor to 49, LOSING the last input frame.
The pipeline then processed misaligned LQ→output frame mapping.

Fix matches naxci1/ComfyUI-FlashVSR_Stable two-stage approach:
1. Pad to next_8n5(N) (next integer >= N of form 8k+5, minimum 21)
2. Add 4 → result is always 8(k+1)+1, a valid 8k+1 — NEVER truncates

Also:
- kv_ratio default 2.0→3.0 (matches naxci1, max quality KV cache)
- local_range default 9→11 (more stable temporal consistency)
- sinusoidal_embedding_1d, precompute_freqs_cis, rope_apply: float32→float64
  (matches naxci1 reference precision for embeddings and RoPE)

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

inference.py

@@ -740,17 +740,22 @@ class FlashVSRModel:
             result = torch.cat([result, pad], dim=0)
         return result
 
+    @staticmethod
+    def _next_8n5(n, minimum=21):
+        """Next integer >= n of the form 8k+5 (minimum 21)."""
+        if n < minimum:
+            return minimum
+        return ((n - 5 + 7) // 8) * 8 + 5
+
     def _prepare_video(self, frames, scale):
         """Convert [F, H, W, C] [0,1] frames to padded [1, C, F_padded, H, W] [-1,1].
 
-        Matches naxci1/ComfyUI-FlashVSR_Stable preprocessing:
+        Matches naxci1/ComfyUI-FlashVSR_Stable two-stage temporal padding:
         1. Bicubic-upscale each frame to target resolution
         2. Centered symmetric padding to 128-pixel alignment (reflect mode)
         3. Normalize to [-1, 1]
-        4. Temporal padding: N+4 then floor to largest 8k+1 (matches naxci1 reference)
-
-        No front dummy frames — the pipeline handles LQ indexing correctly
-        starting from frame 0.
+        4. Stage 1: Pad frame count to next 8n+5 (min 21) by repeating last frame
+        5. Stage 2: Add 4 → result is always 8k+1 (since 8n+5+4 = 8(n+1)+1)
 
         Returns:
             video: [1, C, F_padded, H, W] tensor
@@ -762,6 +767,12 @@ class FlashVSRModel:
         N, H, W, C = frames.shape
         sw, sh, tw, th = self._compute_dims(W, H, scale)
 
+        # Stage 1: pad frame count to next 8n+5 (matches naxci1 process_chunk)
+        N_padded = self._next_8n5(N)
+
+        # Stage 2: add 4 → gives 8(n+1)+1, always a valid 8k+1
+        target = N_padded + 4
+
         # Centered spatial padding offsets
         pad_top = (th - sh) // 2
         pad_bottom = th - sh - pad_top
@@ -769,8 +780,9 @@ class FlashVSRModel:
         pad_right = tw - sw - pad_left
 
         processed = []
-        for i in range(N):
-            frame = frames[i].permute(2, 0, 1).unsqueeze(0)  # [1, C, H, W]
+        for i in range(target):
+            frame_idx = min(i, N - 1)  # clamp to last real frame
+            frame = frames[frame_idx].permute(2, 0, 1).unsqueeze(0)  # [1, C, H, W]
             upscaled = F.interpolate(frame, size=(sh, sw), mode='bicubic', align_corners=False)
             if pad_top > 0 or pad_bottom > 0 or pad_left > 0 or pad_right > 0:
                 # Centered reflect padding (matches naxci1 reference)
@@ -783,17 +795,6 @@ class FlashVSRModel:
             processed.append(normalized.squeeze(0).cpu().to(self.dtype))
 
         video = torch.stack(processed, 0).permute(1, 0, 2, 3).unsqueeze(0)
-
-        # Temporal padding: N+4 then floor to largest 8k+1 (matches naxci1 reference)
-        num_with_pad = N + 4
-        target = ((num_with_pad - 1) // 8) * 8 + 1  # largest_8n1_leq
-        if target < 1:
-            target = 1
-        if target > N:
-            pad = video[:, :, -1:].repeat(1, 1, target - N, 1, 1)
-            video = torch.cat([video, pad], dim=2)
-        elif target < N:
-            video = video[:, :, :target, :, :]
         nf = video.shape[2]
 
         return video, th, tw, nf, sh, sw, pad_top, pad_left
@@ -812,7 +813,7 @@ class FlashVSRModel:
 
     @torch.no_grad()
     def upscale(self, frames, scale=4, tiled=True, tile_size=(60, 104),
-                topk_ratio=2.0, kv_ratio=2.0, local_range=9,
+                topk_ratio=2.0, kv_ratio=3.0, local_range=11,
                 color_fix=True, unload_dit=False, seed=1,
                 progress_bar_cmd=None):
         """Upscale video frames with FlashVSR.