fix(timeline): restore the C++ block length anchors and repair pointer edits

The two BlockCore length setters swapped their anchors relative to the
C++ semantics they document, so the ported edit commands produced wrong
geometry on the live UI paths: roll edits kept the seam still, slides
left negative in-points, and trims wrote the timeline in-point into
media_in (playing the wrong media content).

Adopt three stored-range primitives in block.rs:

- set_length_and_media_out: in fixed, out moves, media untouched
  (resize, trim-out, gaps growing rightward).
- set_length_and_media_in: in fixed, out moves, media_in += old-new
  (resize-with-media-in, splice right half, ripple trim-in).
- set_length_keeping_out (new): out fixed, in moves, media_in +=
  old-new (trim-in body and out-neighbour, slide out-neighbour,
  ripple trim-in of the trailing block).

Point every command at the primitive matching its intent (undopointer,
undogeneral, undoripple, undosplit, graphops, cli, nodeops) and fix the
two real defects the swap hid:

- TrackReplaceBlockWithGapCommand grew a following gap rightward,
  swallowing whatever followed it: the "dragging one clip moves
  unrelated clips" regression. The gap now grows leftward over the
  removed block's span; regression test in domain_test.
- The ripple/splice trims now advance media_in instead of rewriting it,
  and BlockSplitCommand writes both halves' ranges and media
  explicitly (the second half continues from the split point).

Rewrite the KNOWN-SWAP expectations to the correct geometry (roll moves
the seam, slide has no negative in-point, insert-gaps grows rightward,
resize-with-media-in yields media_in = 20) and add the missing media
assertions. TrackSlideCommand documents that the caller positions the
sliding blocks (the stored model has no track layout).

crates/oak-task/src/nodeops.rs

@@ -774,7 +774,8 @@ pub fn block_media_in(project: &ProjectRef, block: NodeId) -> Rational {
 		.unwrap_or_else(|| Rational::new(0, 1))
 }
 
-/// Set the block's length keeping the media out anchored
+/// Set the block's length keeping the in point and the media in point
+/// anchored — the out point (and the media out) shift
 /// (`oaknode_block_set_length_and_media_out`).
 pub fn block_set_length_and_media_out(project: &ProjectRef, block: NodeId, n: i64, d: i64) {
 	let mut guard = lock_project(project);