Turn the lane sideways without changing what it means

The cel sheet is a second projection of the same lane rows: frames run down, lanes run across, and each occupied cell carries the timeline cel's exact selection address. The shared action strip proves the point in the browser test by selecting a cell and issuing the existing hold command.

Before exposing that second entrance, fix the boundary mistakes it revealed. Nested commands now convert the open playhead through their enclosing instance path. Overwrite composes blanking with non-rippling placement as one transaction. Picture-rate and pose sampling select only the generated base frame while hand corrections retain the node's authored frame. Stack validation follows covering replacement layers so a document accepted by the validator cannot throw solely because a later offset sees a different shape.

429 tests, 5,767 assertions; both browser flows; 56 Django tests; optimized frontend build.

docs/lane-model.md

@@ -1,9 +1,9 @@
 # The Lane Model
 
 Revised 2026-09-30. Target design. Cel ownership, source playback, the
-content and cel commands, placement anywhere in a lane, a one-row cel strip
-and the correction-layer evaluator are implemented; the commands that produce a
-correction, overwrite, the retiming commands and the remaining views are not.
+content and cel commands, placement anywhere in a lane, overwrite, a one-row cel
+strip, a frame-down cel sheet and the correction-layer evaluator are implemented;
+the commands that produce a correction and the retiming commands are not.
 See the status note under
 [Proof obligations](#proof-obligations-and-implementation-order).
 
@@ -530,22 +530,23 @@ re-measurement for good; and an incompatible offset threw in the read path,
 which would have taken the stage down on exactly the case the model says to
 report.
 
-Still unbuilt: overwrite as a placement policy (which is now `blank` then
-`place`, composed inside one transaction), slip source, retime, and deleting
-reused content. A lane cannot hold AUDIO cels — `lane-problems`
+Overwrite is `blank` followed by non-rippling placement, composed inside one
+transaction; insertion keeps its ripple rule. Still unbuilt: slip source,
+retime, and deleting reused content. A lane cannot hold AUDIO cels — `lane-problems`
 requires visual ones, though this document says a lane may hold either and
 should reject only a mixture.
 
 What is NOT implemented is a command that produces a layer — the doc's Constant
 adjustment, Ramp and Return motion — and with it the question of how a view
 offers those three over a selected range, and how it offers a conflict for
-resolution. Overwrite, the range and retiming
-commands (blank, trim, move, slip source, retime) and the cel-sheet view
-are also not implemented; a refusal is the current behavior where the model
-demands an explicit choice nobody has made yet. The suite stands at 424 tests
-and 5,749 assertions, with `frontend/test/browser/sequence.mjs` driving the
-editor through create, hold, overflow, undo, reuse, make unique, duplicate,
-split, insert, trim, move and blank. Rewrite tests that encode superseded
+resolution. Slip source and retime are also not implemented; a refusal is the
+current behavior where the model demands an explicit choice nobody has made yet.
+The cel sheet is the same projected cels and selection addresses with its axes
+turned: frames down and lanes across, so commands selected there and in the
+timeline have identical targets. The suite stands at 429 tests and 5,767
+assertions, with `frontend/test/browser/lane.mjs` driving the editor through
+create, hold, overflow, undo, reuse, make unique, duplicate, split, insert,
+trim, move and blank. Rewrite tests that encode superseded
 behavior rather than preserving behavior to keep them green.
 
 Build small adversarial documents and test their domain operations before