feat: add inject_mode (suffix/prefix) to TI pipeline

Observation: n4_baseline loss barely moved (1.025→0.965 over 3000 steps),
token_norm grew linearly without plateau — generator likely ignores last-K
CLIP positions (EOS/padding zone) where suffix injects.

Fix: add inject_mode parameter throughout the pipeline:
- "suffix": replace last K positions (original behavior, model may ignore)
- "prefix": replace positions 1:1+K right after BOS — highest attention
  weight in CLIP, much stronger gradient signal expected

Changes:
- selva_textual_inversion_trainer.py: _inject_tokens() helper centralises
  the torch.cat construction for both modes; used in training loop and eval;
  inject_mode stored in checkpoint files
- selva_textual_inversion_loader.py: reads inject_mode from checkpoint,
  includes in TEXTUAL_INVERSION bundle
- selva_sampler.py: uses _inject_tokens() via bundle's inject_mode field
- selva_ti_scheduler.py: inject_mode in _PARAM_DEFAULTS, config, and
  _train_inner call
- ti_sweep_1.json: updated with prefix_inject group (n4, n8, n4+warm);
  n4_baseline marked completed; suffix experiments retained for comparison

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

nodes/selva_textual_inversion_loader.py

@@ -57,10 +57,14 @@ class SelvaTextualInversionLoader:
             print(f"[TI Loader]   trained {data['step']} / {data.get('steps', '?')} steps  "
                   f"lr={data.get('lr', '?')}", flush=True)
 
+        inject_mode = data.get("inject_mode", "suffix")
+        print(f"[TI Loader]   inject_mode='{inject_mode}'", flush=True)
+
         bundle = {
-            "embeddings": embeddings,   # [K, 1024] float32 on CPU
-            "n_tokens":   n_tokens,
-            "path":       str(p),
-            "init_text":  data.get("init_text", ""),
+            "embeddings":  embeddings,      # [K, 1024] float32 on CPU
+            "n_tokens":    n_tokens,
+            "inject_mode": inject_mode,
+            "path":        str(p),
+            "init_text":   data.get("init_text", ""),
         }
         return (bundle,)