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-node/tests/phase2_units_test.rs

@@ -46,16 +46,27 @@ fn block_core_ranges() {
 	core.set_out(Rational::new(9, 1));
 	assert_eq!(core.length(), Rational::new(4, 1));
 
-	// media_out anchored: in shifts so out stays.
+	// `set_length_and_media_out`: the in point (and the media in point)
+	// stays, the out point follows the length.
 	core.media_in = Rational::new(2, 1);
 	core.set_length_and_media_out(Rational::new(3, 1));
-	assert_eq!(core.out(), Rational::new(9, 1), "out stays put");
-	assert_eq!(core.length(), Rational::new(3, 1));
+	assert_eq!(core.in_(), Rational::new(5, 1), "in stays put");
+	assert_eq!(core.out(), Rational::new(8, 1), "out follows the length");
+	assert_eq!(core.media_in, Rational::new(2, 1), "media in untouched");
 
-	// media_in anchored: in stays, out shifts.
+	// `set_length_and_media_in`: the in point stays while the media in
+	// moves with the length (the media out is what stays anchored).
 	core.set_length_and_media_in(Rational::new(4, 1));
-	assert_eq!(core.in_(), Rational::new(6, 1), "in stays put");
+	assert_eq!(core.in_(), Rational::new(5, 1), "in stays put");
 	assert_eq!(core.length(), Rational::new(4, 1));
+	assert_eq!(core.media_in, Rational::new(1, 1), "media in follows the length");
+
+	// `set_length_keeping_out`: the out point (and the media content end)
+	// stays, the in point shifts.
+	core.set_length_keeping_out(Rational::new(2, 1));
+	assert_eq!(core.out(), Rational::new(9, 1), "out stays put");
+	assert_eq!(core.in_(), Rational::new(7, 1), "in shifts");
+	assert_eq!(core.media_in, Rational::new(3, 1), "media in follows the in point");
 }
 
 /// block.rs: behavior constructors + node inputs.