Write down what the next person needs

`docs/lane-handoff.md`, after `timing-handoff.md`'s shape, because the next
thread starts cold and the expensive part of that is not the code — it is the
decisions that were argued out and would otherwise be argued again.

So the section that matters most is the one listing what NOT to re-litigate:
the shot length is authored, placing ripples and overwrite is blank-then-place,
a position inside a cel refuses and names split, a correction has no time space
of its own, a layer's values are a channel, a conflict is not a problem, and a
command refuses rather than guesses. Each of those is a paragraph here and a
commit message in full.

Then the vocabulary, since it was settled one commit ago and the old words are
still in this repository's history: instance, cel, lane, drawing, and placement
for where a node sits only. With the warning that `exposure` still means the
`:expose` grid and always did.

Then the mechanisms that keep paying out — `:span` in the node's own frames
above all, which is why split, trim and blank cost almost nothing — the known
gaps, and how to run the suites, including that a release build clobbers the dev
bundle the browser tests need.

Recommended next piece is the cel sheet: it needs no new model, and it is the
first real evidence the document is not shaped by the timeline that grew up
with it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/lane-model.md

@@ -7,6 +7,9 @@ correction, overwrite, the retiming commands and the remaining views are not.
 See the status note under
 [Proof obligations](#proof-obligations-and-implementation-order).
 
+[Lane and cel handoff](lane-handoff.md) records what is built, the decisions
+that are settled, and what to do next.
+
 This revises the Claude artifact [The Lane Model](https://claude.ai/code/artifact/cd42981d-ed08-493f-94df-b7dd6657f0e6).
 Its prose and diagram source were recovered from session
 `1c603f71-84eb-498e-aeaf-4c0346f1f513`; the live artifact was not accessible for