timeline: adjustment layers and first-class transitions

Adjustment layers (docs/zh/plans/adjustment-layers-and-transitions.md):
a new timeline block type whose effect chain grades the composite of
every video track below it, over its own range (spanning clips or a
slice of one). The graph path flushes the lower tracks at the block's
track boundary and sweeps the composite through the chain via a
transient texture-source node; the montage path mirrors it with
AdjustmentSpan tickets (wire-compatible), so worker previews and
exports agree. An empty-area context menu creates one; the block
trims/moves/deletes like a clip, with undo everywhere.

Transitions: seam blocks come alive - cross dissolve/fade/wipe/slide
evaluate both neighbors through the graph path with progress from the
transition's own range (never the whole clip). Ctrl+Shift+D or the clip
menu inserts a default transition; the gpui wedges render and drag to
resize offsets undoably, and TransitionRemoveCommand now restores
offsets and edges on undo. The transitionfx node form runs the same
shaders on an adjustment layer with progress_in auto-filled from the
layer's span (explicit value wins).

Also: every built-in effect name and parameter name is now
translatable (360 node.* keys per locale, zh-CN fully translated, two
coverage tests guard future gaps); the new nodes register in
nodes/mod.rs with the factory smoke table updated; textfootage and
adjustment-layer i18n keys included.

crates/oak-cli/src/engine.rs

@@ -657,6 +657,7 @@ pub fn render_frame(
 		cache_timebase: None,
 		footage: None,
 		montage,
+		adjustments: Vec::new(),
 	};
 	let id = m.tickets.next_id();
 	m.tickets.submit_video_with_id(id, params, Box::new(|_| {}));