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.

docs/examples.md

@@ -52,7 +52,7 @@ contact (#161) the ring will be pressed by contact against a groove.
 Air (ρ = 1.2 kg/m³, μ = 1.8 × 10⁻⁵ Pa s) at 25 mm/s past the same L-bracket standing on the
 floor, the upright's back to the wind. A 240 × 120 × 100 mm box of 2.5 mm cells (184 320),
 uniform inlet, outlet at zero pressure, a no-slip floor and slip walls at the sides and top;
-D3Q19 lattice Boltzmann (TRT) for 16 s, about 6.7 flow-throughs, ten frames. Reynolds number
+D3Q19 lattice Boltzmann (TRT) for 16 s, about 1.7 passes through the 240 mm box, ten frames. Reynolds number
 83 on the upright's 50 mm height: laminar, a steady wake. About 10 s.
 
 **Look at:** the step's report (`solve.fluid`, or Results after Solve): the force on every
@@ -72,6 +72,14 @@ velocity magnitude on a slice through the middle of the box (`results.slice`; pr
 `field: "pressure"`), the bracket coloured by the flow beside it (`results.surface`) and
 streamlines seeded upstream of it, coloured by speed (`results.streamlines`).
 
+![The solved flow: velocity on the mid-slice, a slow wake behind the upright and faster flow over it, with streamlines bending around the bracket](media/examples/flow-past-flow.png)
+
+**Watch it start up:** a fluid step animates its frames (Results ▸ Animate, or `results.animate`),
+playing the start-up transient from the uniform 25 mm/s initial field to the settled wake — the
+wake forming behind the upright and the drag settling over the 16 s run. A clip is at
+[flow-past-transient.mp4](media/examples/flow-past-transient.mp4), regenerated by
+[tools/flow-clip](../tools/flow-clip/README.md).
+
 ## Heated bracket (`heated-bracket`)
 
 ![The steel bracket's temperature, 120 °C at the heated end of the upright falling to 95 °C at the foot](media/examples/heated-bracket.png)