fix(render3d): outline_model's batch survives the whole frame

A frame is drawn by more than one pass - a shadow map, a water reflection, the
scene - and actor_draw runs in each. An Actor's rim survives that because it is
a field on the actor; the queue did not, because the first pass to run emptied
it. A queued outline was drawn into whichever target came first and was gone by
the time the scene was drawn, so nothing appeared with every uniform, matrix
and mesh correct - which took a while to find.

A flush now closes the batch rather than clearing it, and the next
outline_model opens a new one.

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

changes/outline-queue-lifetime.md

@@ -0,0 +1,12 @@
+bump: patch
+type: fix
+**`outline_model`'s batch survives the whole frame.** A frame is drawn by more than one
+pass — a shadow map, a water reflection, the scene — and `actor_draw` runs in each of
+them. An Actor's rim survives that because it is a field on the actor; the queue did not,
+because the first pass to run emptied it, so a queued outline was drawn into whichever
+target happened to come first and was gone by the time the scene was drawn. Nothing
+appeared, with every uniform, matrix and mesh correct.
+
+A flush now *closes* the batch rather than clearing it, and the next `outline_model`
+opens a new one: every pass in a frame sees the same requests, and a caller still needs
+no frame hook.