Paint: vector background cels on the kept frames

A sketch, and labelled as one. It exists to test whether the aesthetic holds
when a human draws the background rather than the tracker deriving it, and it
is meant to be replaced by a real paint surface with onion skin and undo. Kept
to one dependency-free module so throwing it away is a delete, not surgery.

Cels are drawn on the frames that get their own drawing and hold until the
next one - the same rule the plate follows, and literally the same lookup, so
the two cannot disagree about what is on screen. Editing always targets the
cel you can see, so you can scrub anywhere and keep drawing.

Pen places vertices and closes on the first one. Edit drags vertices or whole
shapes, shift-click inserts, alt-click removes. Layers stack front-at-top with
per-layer colour, show/hide and reorder. Drag a strip thumbnail onto the canvas
to seed this cel from that one, every layer, as a deep copy - sharing the point
arrays would make two cels silently edit each other.

Two rules are enforced rather than left to discipline:

Colours are PALETTE INDICES, never RGB. Sampling colour from the source is the
one move docs/design.md calls irrecoverable, and a paint tool is exactly where
that discipline would leak, so the picker cannot express a colour outside the
ramp.

Vertices snap to the 320x200 grid. On a hard-edged indexed rasteriser a shape
nudged by 0.4px moves an edge by a whole pixel or not at all depending on where
it happens to land, so sub-pixel vertices shimmer instead of holding still.

Drawings autosave to localStorage per take name. They are the only thing here a
person made by hand; everything else regenerates. Not in the .take export yet.

Also adds serve.py, a no-store dev server. python3 -m http.server sends
Last-Modified and browsers cache ES modules on it hard enough that a reload
serves a stale app.js against a fresh index.html: the new knobs appear, nothing
wires them, no error fires, and it reads as "your feature does not work". That
cost real time this session.

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

serve.py

@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+"""Dev server that refuses to cache anything.
+
+`python3 -m http.server` sends Last-Modified, and browsers cache ES modules on
+it hard enough that a reload can serve a stale js/app.js against a fresh
+index.html. That failure is silent and points nowhere near its cause: the new
+knobs appear in the markup, nothing wires them, no error is raised, and the
+symptom reads as "the feature you just added does not work".
+"""
+import sys
+from http.server import SimpleHTTPRequestHandler, ThreadingHTTPServer
+
+
+class NoCache(SimpleHTTPRequestHandler):
+    def end_headers(self):
+        self.send_header('Cache-Control', 'no-store, must-revalidate')
+        self.send_header('Expires', '0')
+        super().end_headers()
+
+    def log_message(self, fmt, *args):
+        if '304' not in fmt % args:
+            super().log_message(fmt, *args)
+
+
+if __name__ == '__main__':
+    port = int(sys.argv[1]) if len(sys.argv) > 1 else 8777
+    print(f'http://127.0.0.1:{port}  (no-store)')
+    ThreadingHTTPServer(('127.0.0.1', port), NoCache).serve_forever()