Become arthur: a standalone suite, not an Animator Pro front-end

The test renderer turned out to be the product. Everything that decides how the
work looks - stabilisation, reduction, timing, frame removal, palette - already
happens here, and the flat indexed output already reads the way it should.

The reason to leave is in the original design's own rule: never make a timing
decision that requires a full render to evaluate. Honouring that moved every
judgement out of Animator Pro, which left the host doing nothing but writing a
file, in exchange for modal UI, minutes-long renders, one-level undo, FLX delta
invariants, a single tween state and a cel singleton.

What does NOT change is the constraint. 320x200, indexed palette, flat fills,
no antialiasing - inherited, but load-bearing rather than accidental. The
rasteriser writes palette indices and expands to RGBA only at the end precisely
so nothing can soften an edge. Modern conveniences belong in the workflow.

Adds docs/design.md: the principles, carried over without the Poco/FLX/cel
machinery, plus architecture and an honest list of what is missing - the
largest gap being that plates still have nowhere to be drawn.

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

js/selftest.js

@@ -2,7 +2,7 @@
 // module graph the tool uses is what gets tested.
 //
 // The ring-simplicity check exists because "fixed topology" is load-bearing in
-// docs/roto-puppet.md: because hold parts CUT between poses rather than
+// docs/design.md: because hold parts CUT between poses rather than
 // interpolating, a ring whose vertex order is wrong self-intersects and renders
 // as blocks meeting at corners. It is invisible at some vertex counts and obvious
 // at others, so it needs an assertion rather than an eyeball.