Port steps 0-1: scaffold, the oracle, and the pure bottom

Scaffolds frontend/ (shadow-cljs, reagent 1.2.0, re-frame 1.4.3) and ports
everything below the data model, with the JS kept as a numeric oracle.

  domain/landmarks  index tables, verbatim
  domain/ring       subsample, offset, simplicity
  domain/geom       similarity fit, procrustes, moving average
  domain/raster     indexed scanline fill, stencil, disc, rect
  domain/palette    the ramp, and the no-sampled-RGB rule

58 tests, 166 assertions. Parity with js/ on the identical 72-frame synthetic
track: fit-similarity, procrustes-mean, fit-residual, moving-average,
smooth-transforms, offset-ring and subsample-slots to 1e-9; the raster
pixel-for-pixel over the whole buffer.

Three deviations from the JS, each for a reason:

- synth.cljs jitters from a SEEDED generator, not Math.random. Parity is only
  checkable if both sides can be handed the same track, and a failing assertion
  has to be reproducible. `:rand-fn` takes the generator over, so oracle.mjs
  stubs js/Math.random and js/ itself stays untouched.

- raster/->rgba replaces toImageData. ImageData is a DOM type and domain/ may
  not touch the DOM; returning plain bytes also lets the
  no-intermediate-colours assertion run in node. ui/canvas wraps it later.

- offset-ring lives in domain/ring, not domain/geom, per architecture.md: it is
  an operation on an ordered traversal, not on a transform.

Step 1's "done" also names the swapped-iris vote, but pairIrises is in
pipeline.js and belongs to step 7. The precondition is asserted instead --
`:swap-iris` really does move both blocks -- so the vote will have a track that
disagrees with it when it arrives.

One finding, recorded in full in the test that measures it: smooth-transforms
buys nothing on the synthetic track. Against jitter-free ground truth, radius 1
helps by 17% on one noise realisation and hurts by 0.5% on another, so its
benefit is within noise; from radius 2 up the cost is unambiguous, and by radius
5 the filter is below the true motion's own high-frequency energy, i.e.
smoothing away performance. The test pins the shape of the knob rather than a
preferred value. This may say more about the synth's jitter being unrealistically
small (+/-0.001 normalised) than about the knob; step 6 settles it on real
footage.

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

frontend/README.md

@@ -0,0 +1,76 @@
+# frontend
+
+The ClojureScript half. See `docs/port-plan.md` for what is being built and in
+what order; this file is only how to run it.
+
+## Once
+
+```sh
+mise install          # from the REPO ROOT: java 21+, node 20, clojure, python
+cd frontend && npm install
+```
+
+`java` must be 21+. On an older JDK shadow-cljs fails with "CompilerOptions has
+been compiled by a more recent version of the Java Runtime", which reads like a
+shadow-cljs bug and is not one. `mise install` is what prevents it.
+
+## The tests
+
+```sh
+cd frontend && npm test
+```
+
+That is three things in order: regenerate the JS oracle's answers, compile the
+`:test` build, run it under node.
+
+```
+node test/parity/oracle.mjs      # drives js/ and writes test/parity/oracle.json
+shadow-cljs compile test
+node out/node-tests.js
+```
+
+Run them separately if a compile error is in the way. The oracle JSON is
+generated, not committed.
+
+## The app
+
+```sh
+cd frontend && npx shadow-cljs watch app
+```
+
+Then open **<http://localhost:8778/index.html>** — with the `/index.html`, not
+bare `/`. This shadow-cljs does no directory-index resolution, so `/` is a 404
+whatever the roots are.
+
+Through step 1 the page is a placeholder on purpose: there is nothing to render
+until the data model exists, and a shell built before the model is a shell built
+around a guess. Something moves on screen at step 2.
+
+Port 8778 is deliberately not 8777. `python3 serve.py` from the repo root still
+runs the old JS tool on 8777, and the two are meant to run side by side — that is
+the whole reason `js/` is still in the tree.
+
+From step 9 Django serves the page and `:dev-http` goes away.
+
+## The oracle
+
+`js/` is the numeric oracle, not dead weight. `test/parity/` runs both
+implementations on the same synthetic track and diffs them: `fit-similarity` and
+`procrustes-mean` agree to 1e-9, the raster pixel-for-pixel.
+
+Both sides get the identical track because `js/synth.js` reads `Math.random` at
+call time, so `oracle.mjs` stubs it to a constant and the CLJS side passes
+`:rand-fn (constantly 0.5)`. `js/` itself is never modified.
+
+**`test/parity/` and `arthur.parity-test` get deleted in one commit at step 5.**
+A parity test pins behaviour while code moves; keeping it afterwards would bake
+the prototype's mistakes into the rewrite and make them permanent.
+
+## Layout
+
+```
+src/arthur/domain/    pure. No re-frame, no DOM, no flow/.
+test/arthur/synth.cljs  the synthetic track — test infrastructure, not src
+test/parity/            the JS oracle harness. Deletable at step 5.
+public/index.html       dev host page. Django replaces it at step 9.
+```