Initial commit: video effects processing system

Add S-expression based video effects pipeline with modular effect
definitions, constructs, and recipe files.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

effects/strobe.py

@@ -0,0 +1,90 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = ["numpy"]
+# ///
+"""
+@effect strobe
+@version 1.0.0
+@author artdag
+
+@description
+Strobe / Posterize Time effect. Locks video to a reduced frame rate,
+creating a choppy, stop-motion look. Also known as frame hold.
+
+@param frame_rate float
+  @range 1 60
+  @default 12
+  Target frame rate in fps. Lower = choppier.
+
+@param sync_to_beat bool
+  @default false
+  If true, hold frames until next beat (overrides frame_rate).
+
+@param beat_divisor int
+  @range 1 8
+  @default 1
+  Hold for 1/N beats when sync_to_beat is true.
+
+@state held_frame ndarray
+  Currently held frame.
+
+@state held_until float
+  Time until which to hold the frame.
+
+@example
+  (effect strobe :frame_rate 8)
+
+@example
+  ;; Very choppy at 4 fps
+  (effect strobe :frame_rate 4)
+
+@example
+  ;; Beat-synced frame hold
+  (effect strobe :sync_to_beat true :beat_divisor 2)
+"""
+
+import numpy as np
+
+
+def process_frame(frame: np.ndarray, params: dict, state: dict) -> tuple:
+    """
+    Apply strobe/posterize time effect to a video frame.
+
+    Args:
+        frame: Input frame as numpy array (H, W, 3) RGB uint8
+        params: Effect parameters
+            - frame_rate: target fps 1-60 (default 12)
+            - sync_to_beat: use beat timing (default False)
+            - beat_divisor: beat fraction (default 1)
+        state: Persistent state dict
+            - held_frame: currently held frame
+            - held_until: hold expiry time
+
+    Returns:
+        Tuple of (processed_frame, new_state)
+    """
+    target_fps = max(1, min(params.get("frame_rate", 12), 60))
+    sync_to_beat = params.get("sync_to_beat", False)
+    beat_divisor = max(1, int(params.get("beat_divisor", 1)))
+
+    # Get current time from params (executor should provide this)
+    t = params.get("_time", 0)
+
+    if state is None:
+        state = {}
+
+    # Initialize state
+    if "held_frame" not in state:
+        state["held_frame"] = None
+        state["held_until"] = 0.0
+        state["last_beat"] = -1
+
+    # Frame rate based hold
+    frame_duration = 1.0 / target_fps
+
+    if t >= state["held_until"]:
+        # Time for new frame
+        state["held_frame"] = frame.copy()
+        state["held_until"] = t + frame_duration
+
+    return state["held_frame"] if state["held_frame"] is not None else frame, state