docs: a flow-past gallery clip and solved still

Add the start-up-transient clip and a solved still of the flow-past sample to
the examples gallery: air flowing past the L-bracket, the wake forming behind
the upright and the drag settling over the 16 s run (it does not shed; that
needs a finer grid or a collision operator stable near the lattice's stability
limit, #557).

tools/flow-clip/make.py regenerates them: it drives the desktop app over the
control channel, solves the lattice Boltzmann step with a dense frame output,
renders the model-only viewport at each frame with a fixed legend clamp
(results.frame then ui.render), and encodes an MP4 and a GIF with ffmpeg, each
frame captioned with the elapsed flow time. The geometry is the sample's own
upright-bracket recipe; nothing comes from a commercial product.

Part of #164.

ROADMAP.md

@@ -130,3 +130,13 @@ Temperature is available on fluid slices, obstacle surfaces and streamline colou
 saved frames carry it. Signed face-average wall heat follows recorded histories and the
 model's power/area units. Commands, the Results panel and headless pictures share the view;
 initial frames without recorded wall heat remain uncoloured.
+
+### Flow start-up transient and gallery clip (#164)
+
+`results.animate` now plays a fluid step's frames in order, the start-up transient from the
+uniform initial field to the settled flow, the way an explicit step plays its frames; the fluid Results
+panel gains an Animate control. The `flow-past` gallery sample gains a clip and a solved still of
+the air flowing past the L-bracket, regenerated by `tools/flow-clip/make.py` (the app rendered
+frame by frame over the control channel, encoded with ffmpeg). The wake forms and the drag
+settles; it does not shed (that needs a finer grid or a collision operator stable near τ⁺ = 0.5,
+#557).