Vector art has a tab to draw in, which is stage 3 of #129

Stages 1 and 2 gave the asset a model and a picture, but the only way
to put a shape into it was a test calling mutate.  The tab closes that
gap: what you draw there is what the sprite in the scene carries, with
no extra step, because the fingerprint changes and the picture registry
of #185 redraws.

- The tab has a header with Select, Pen, Grid and Fit, a chequerboard
  behind the canvas and the canvas itself as a frame.  It paints with
  the same commands as the rasterizer, paintDrawing being exported
  from viewport/vectorArtPicture.ts for it, so the tab cannot drift
  away from the picture.
- Pen sets points on a click and pulls the handles out on a drag, both
  of them mirrored; a click on the first point closes the ring and Esc
  ends an open line.
- Select drags a point, a handle or a whole shape, a click on an edge
  puts a point there, a double click takes one away, and Alt turns a
  corner round and back.  Delete removes the selected shape.
- The rules grew inside polygonEdit.ts instead of a second module, as
  the plan says, so a collider polygon and a frame outline get the
  same behaviour where it applies.
- A point on a curved edge splits the curve by de Casteljau instead of
  cutting it: both halves together draw exactly what was there before.
- A curved edge is walked instead of measured over its chord, or a
  click on the bow of a curve would grab nothing; the same walk carries
  the test for the inside of a ring.
- One gesture is one undo step.  The preview writes the model directly
  and the old state is put back on release before the command runs,
  which is the pattern the frame outlines of #141 already use.
- state.ts gained a selection kind "shape".  The inspector shows the
  rows of the asset for it, the shape list with colour, opacity and
  stroke being stage 4.

One counter-check was no counter-check: disabling the pen's closing
tolerance by multiplying it with 0 left the run green, and rightly so,
because the closing click lands on exactly the screen coordinate point
0 came from, so the distance is 0 and <= 0 is true.  Only an
unsatisfiable condition really switched it off, and then 2 checks fail.
A second trap on the way: a false && in front of the guard breaks the
TS narrowing, npm run build bails out, and two runs went against the
old bundle.

Tests: check.sh 127 files, 1878 tests, EXIT 0 (was 126 / 1860).
Full e2e suite ALL PASSED - 1172 checks, module 47 alone 17 (was 10).
18 new unit tests in tests/editor/ring-edit.test.ts and 7 new e2e
checks in blocks 47c and 47d.  Two counter-checks hold: no de
Casteljau fails 1 unit test, measuring a curve over its chord fails
another.  Originals checked back with cmp and rebuilt.

README.md

@@ -43,7 +43,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1165 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1172 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs