render: apply clip effect stacks in the montage path

Adding an effect to a clip did nothing: the sequence render is
flattened into a montage (decode + composite), and MontageClip carried
no effect data at all.

- MontageClip gains an ordered effect stack (type id / enabled /
  effect input / parameter values); protocol v2 carries it as an
  additive wire field (older peers default to an empty stack).
- renderops::video_montage fills the stack from the effect chain
  (the footage source node — the chain end without an effect input —
  is dropped; the montage decodes the footage itself). Export
  (oak-task) and the multicam single-track montage fill it too.
- The worker applies the stack between decode and composite: built-in
  Opacity gets a CPU evaluator (C++ opacity.frag parity — whole vec4,
  alpha included, unity pass-through); everything else dispatches as an
  OFX plugin job through a new instance-factory slot (oak-plugin
  lazily creates + caches one instance per identifier per render
  process) with the montage's parameters injected. Disabled effects
  bypass (the C++ traverser pushes the effect input through). Unknown
  types warn once per type id and pass through — no silent no-ops.

Not covered (explicitly): Transform/Crop and the other ~30 built-in
effects have no CPU evaluator in oak-render (they pass through with a
warning), keyframed parameter animation, audio effect chains, and the
CLI's simplified montage.

Acceptance: a real 50% Opacity on real media quarters the rendered
pixels both in-process (renderops test) and through a real worker
process over IPC + shared memory (procpool_integration test);
disabling restores the plain render byte-for-byte.

crates/oak-cli/src/engine.rs

@@ -535,6 +535,9 @@ pub fn video_montage(p: &ProjectRef, seq_id: NodeId, time: Rational) -> Vec<Mont
 					out_time: out,
 					media_in: clip.core.media_in,
 					gain: 1.0,
+					// The CLI's simplified montage builder does not resolve
+					// effect chains (unchanged behavior).
+					effects: Vec::new(),
 				});
 			}
 		}
@@ -585,6 +588,7 @@ pub fn audio_montage(p: &ProjectRef, seq_id: NodeId, range: TimeRange) -> Vec<Mo
 					out_time: out,
 					media_in: clip.core.media_in,
 					gain: 1.0,
+					effects: Vec::new(),
 				});
 			}
 		}