irxmanim: short animations with no dependencies and no script

[tamnd]

The animation library from #10. It takes manim's vocabulary, mobjects and a
scene and play for one beat of it, and none of manim's implementation:
manim renders video, which wants cairo, ffmpeg and usually a LaTeX install,
and none of that fits in the first cell of a Colab notebook or survives
being saved into one. What comes out of here is one inline <svg> with a
stylesheet in it.

from irxmanim import Scene, Code, fade_in, fade_out, highlight, move

s = Scene(430, 208, caption="opt -passes=dce deletes the instruction nothing reads")
code = s.add(Code(x=36, y=58, lines=before))
s.play(fade_in(code))
s.play(highlight(code[2]))
s.play(fade_out(code[2]))
s.play(move(code[3], dy=-17), move(code[4], dy=-17))

Mobjects: Box, Code, Caption, Arrow with link for an arrow between
two of them, Group, and the Wash a highlight fades in. Code is a group
with one mobject per line, so a pass can be shown doing what a pass does,
which is to touch one instruction and leave the rest alone.

Animations: fade_in, fade_out, move, highlight, pulse, draw.

Print a scene and you get the storyboard, which is the form a review reads.
Show it in a notebook and you get the picture. svg() writes a file and
frame(t) gives one still with nothing moving in it.

What this closes off, and why

• CSS keyframes, no script. The output goes into saved notebooks, into
  Colab's output sandbox and into static HTML on the site, and script
  survives none of those reliably. It also means the picture animates when
  it is loaded as an ordinary <img>, which script would not.
• The markup is the last frame. Elements are written out where they end
  up and the animation walks back to the beginning and forward again. A
  renderer that drops the stylesheet, a printed page, and anybody whose
  system asks for less motion all get the finished picture rather than an
  empty box. prefers-reduced-motion: reduce turns the animation off
  rather than being ignored.
• Easing is sampled here, not handed to CSS. A keyframe timing function
  applies per stop, and one mobject's stops get interleaved with another's,
  which distorts the curve quietly.
• Ninety seconds is the cap, the one in CONTRIBUTING.md. A scene over
  it raises rather than rendering.

Also in here

irx.ir grows pieces(), the tokeniser loop that irx.ir.highlight and
irx.graphs._tspans each had their own copy of. Three things render IR now
and they agree about what a keyword is.

docs/diagrams/pass-step.svg is the first scene, wired into build.py diagrams like the others, and byte stable across rebuilds because the one
generated id in it is a content hash. The IR in it is what opt -passes=dce -S printed, and the transcript is in the script that draws it.

Checks

• toolkit: 244 tests, OK (35 new, none of which need a compiler)
• repo: 98 tests, OK
• ruff check ., build.py check, build.py diagrams twice with no diff,
  prosecheck, refcheck --ledger clean

Part of #10.