feat(cnn_v3/training): add --single-sample option + doc fixes

- train_cnn_v3.py: --single-sample <dir> implies --full-image + --batch-size 1
- cnn_v3_utils.py: CNNv3Dataset accepts single_sample= kwarg (explicit override)
- HOWTO.md: document --single-sample workflow, fix pack_photo_sample.py usage (--target required)
- HOW_TO_CNN.md: fix GBufferEffect seq input (prev_cnn→source), fix binary name (demo→demo64k), add --resume to flag table, remove stale "pack without target" block

handoff(Gemini): --single-sample <dir> added to train_cnn_v3.py; docs audited and corrected

1 files changed, 14 insertions, 11 deletions

diff --git a/cnn_v3/training/cnn_v3_utils.py b/cnn_v3/training/cnn_v3_utils.py
index bef4091..50707a2 100644
--- a/cnn_v3/training/cnn_v3_utils.py
+++ b/cnn_v3/training/cnn_v3_utils.py
@@ -286,7 +286,8 @@ class CNNv3Dataset(Dataset):
                  channel_dropout_p: float = 0.3,
                  detector: str = 'harris',
                  augment: bool = True,
-                 patch_search_window: int = 0):
+                 patch_search_window: int = 0,
+                 single_sample: str = ''):
         self.patch_size          = patch_size
         self.patches_per_image   = patches_per_image
         self.image_size          = image_size
@@ -296,16 +297,18 @@ class CNNv3Dataset(Dataset):
         self.augment             = augment
         self.patch_search_window = patch_search_window
 
-        root       = Path(dataset_dir)
-        subdir     = 'full' if input_mode == 'full' else 'simple'
-        search_dir = root / subdir
-        if not search_dir.exists():
-            search_dir = root
-
-        self.samples = sorted([
-            d for d in search_dir.iterdir()
-            if d.is_dir() and (d / 'albedo.png').exists()
-        ])
+        if single_sample:
+            self.samples = [Path(single_sample)]
+        else:
+            root       = Path(dataset_dir)
+            subdir     = 'full' if input_mode == 'full' else 'simple'
+            search_dir = root / subdir
+            if not search_dir.exists():
+                search_dir = root
+            self.samples = sorted([
+                d for d in search_dir.iterdir()
+                if d.is_dir() and (d / 'albedo.png').exists()
+            ])
         if not self.samples:
             raise RuntimeError(f"No samples found in {search_dir}")