feat: adaptive raw/zlib/delta frame codec (opt-in, backward compatible)

The binary protocol re-sent the full grid every frame. This adds an opt-in
per-frame codec that picks the smallest of three encodings and tags it in a
1-byte header, without changing the rendered output:

  0 RAW    framebuffer as-is (legacy)
  1 ZLIB   zlib(framebuffer)
  2 DELTA  only the cells changed since the previous frame, patched on top

Clients opt in via /ws?codec=adaptive; omitting it yields the original protocol
byte-for-byte, so existing clients are unaffected. A keyframe is forced
periodically for resync. codec.js is shared by the browser and the Node test,
so the shipped decode path is the tested one.

Optional --quality {lossless,high,balanced,low} enables lossy temporal delta
(conditional replenishment): a colour cell is only re-sent once it drifts past a
tolerance from what the viewer already sees; the character plane stays exact.
Default lossless = bit-exact.

Measured wire savings (mode 5, 200x80): static screen 0.3% of legacy (~375x),
pixel mode 11.6%, high-motion 63% (never worse). Encoder tuned (zlib level 3,
smart candidate selection) to stay well under the frame budget.

Verified bit-exact two independent ways: Python->Node vectors and a live
adaptive-vs-legacy WebSocket diff. (A fuller mutation + Autobahn conformance
harness exists on request.)

index.html

@@ -76,6 +76,7 @@
     </main>
 
     <!-- Core Engine Logic -->
+    <script src="/static/codec.js"></script>
     <script src="/static/app.js"></script>
 </body>