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.

tools/flow-clip/README.md

@@ -0,0 +1,39 @@
+# Flow-past clip
+
+`make.py` regenerates the gallery clip and still of the `flow-past` sample (docs/examples.md):
+`docs/media/examples/flow-past-transient.mp4` (the start-up transient of air flowing past the
+L-bracket, from the uniform 25 mm/s initial field to the settled wake) and
+`docs/media/examples/flow-past-flow.png` (a clean iso render of the settled flow).
+
+```sh
+python3 tools/flow-clip/make.py                   # build, run, encode into docs/media/examples
+python3 tools/flow-clip/make.py --no-build        # with the release binaries already built
+python3 tools/flow-clip/make.py --frames /tmp/f   # also keep the rendered pictures
+python3 tools/flow-clip/make.py --gif             # also write a GIF (not committed; larger)
+```
+
+Needs Python 3 (standard library only), a display for the FieldCraft window (the pictures are
+drawn offscreen with `ui.render`, so a locked or virtual display is fine — on a headless machine
+run it under `xvfb-run`), and `ffmpeg` with libx264 on the `PATH` (or `FFMPEG`). It honours
+`CARGO_TARGET_DIR`; set `FIELDCRAFT_GPU=software` for a software renderer.
+
+What it does:
+
+1. Builds `fieldcraft` and `fieldcraft-cli` (release) and starts the app with `--control PORT`.
+2. Opens the `flow-past` sample and, so the transient plays smoothly, edits its fluid step to
+   store `FLOW_CLIP_FRAMES` frames (48 by default) over the 16 s run (`model.step.edit`), then
+   solves the lattice Boltzmann step on the CPU (`solve.fluid`).
+3. Fixes the velocity legend clamp (0 to 45 mm/s) so the colours mean the same thing from frame
+   to frame, sets a light theme and an iso view, and renders the model-only viewport at each
+   field frame (`results.frame` then `ui.render`): velocity on the mid-slice, the bracket
+   coloured by the flow beside it, and streamlines seeded upstream, coloured by speed.
+4. Captions each frame with the elapsed flow time in a band below, drawn by the demo-video tool's
+   standard-library TrueType caption drawer (`tools/demo-video/caption.py`, Inter SemiBold).
+5. Encodes the frames at 12 fps (the settled wake held at the end) into the MP4 (H.264) and writes
+   the settled still. `--gif` also writes a GIF (800 px, its own palette); it is not committed
+   because it is several times larger than the MP4.
+
+The clip shows the wake forming behind the upright and the drag settling; it does not shed, which
+needs a finer grid or a collision operator stable near τ⁺ = 0.5 (see issue #557). The geometry is
+the original upright-bracket SolidCraft recipe (the `flow-past` sample); nothing comes from a
+commercial product.