fix(ltx): Soften Schedule — warn once when patched but no usable sigmas

PR #266 review finding #2. When the hook has no reference sigma from the
run AND none wired to the 'sigmas' input, it returns the original mask —
correct, but silent, so a patched node on a sampler path that doesn't
pass sigmas looked disabled with no signal. Emit a single warning
("soften inactive this run — connect SIGMAS") the first time it happens.

Behavior unchanged (still a clean no-op); this only surfaces the state.
Tests unchanged (21/21).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

k_ltx_keyframe_soften_schedule.py

@@ -178,6 +178,7 @@ class LTXKeyframeSoftenSchedule:
         except Exception:
             connected_ref = None
         logged = [False]
+        warned = [False]
 
         def denoise_mask_function(sigma, denoise_mask, extra_options=None):
             if prev_fn is not None:
@@ -202,6 +203,16 @@ class LTXKeyframeSoftenSchedule:
             if ref is None:
                 ref = connected_ref
             if not ref or ref <= 0.0:
+                # No usable reference sigma from the run and none wired: the node
+                # is patched but inert this run. Warn ONCE so a silent no-op on a
+                # sampler path that doesn't pass sigmas doesn't look like a bug.
+                if not warned[0]:
+                    warned[0] = True
+                    log.warning(
+                        "[LTXKeyframeSoftenSchedule] no usable sigmas from the run "
+                        "and none wired to the 'sigmas' input — soften inactive this "
+                        "run. Connect your sampler's SIGMAS to enable it.",
+                    )
                 return denoise_mask
 
             try: