Decompress a full-screen picture and six frames of video from strings

The megademo gains a scene: an 800x600 vaporwave sunset carried inside
the listing, then six looping delta frames of full motion video -- the
floor grid rolling forward and the sun's slices crawling, about 220
bytes a frame. DATA cannot carry an image (512 items, ~350 owned by the
stroke font), so the picture rides in RLE-encoded string literals
decoded with INSTR: 2 KB of base frame, 1.3 KB of video. Row records
jump the decoder between changed rows, an unreachable colour is the
skip, black draws so a delta can erase, and the rays are struck live
over every frame rather than encoded. The 63-byte bird gliding over it
is the one image DATA does have room for, via SPRSAV's type-in form.

examples/megademo/vaporwave.py composes the image in palette space,
dithers by row so the gradients band instead of shattering the RLE,
proves base-plus-deltas reproduces every frame against a simulation of
the decoder, and owns the generated block between the PICTURE markers.
TODO.md section 4 records the DATA ceiling and the wanted verb.

Co-Authored-By: Claude Code (Fable 5) <noreply@anthropic.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EwxGB6TdoVvZ11KQQME9cL

TODO.md

@@ -778,6 +778,17 @@ and that is the argument for the verb. Ordered by how much BASIC each one would
   the note rather than read from shared state at release. The demo works around it the way
   1985 did on one channel: an interleaved kick–bass–arpeggio line (the `TUNE` label), which
   is authentic and should not be mandatory.
+- **`DATA` is too small a pipe for assets, and `SPRSAV`'s type-in form is 24 by 21.** The
+  pool is 512 items and the demo's stroke font already holds ~350, so a full-screen picture
+  can never arrive by `DATA` — and the one sprite form that reads from the program is fixed
+  at the VIC-II's 24x21 monochrome. The demo's scene 5 shows both the ceiling and the way
+  around it: a 160x120 sixteen-colour image rides in **string literals** instead — RLE, two
+  characters a run, ~4 KB in eighteen `IM$()` assignments, decoded with `INSTR` and painted
+  in 5x5 blocks (`examples/megademo/vaporwave.py` generates the block) — while the 63-byte
+  bird beside it is all `DATA` can actually carry. Strings are the workaround, not the
+  answer: every program that wants a picture must also carry a decoder. What is wanted is
+  either a bigger `DATA` pool (it is one constant) or, better, a `SPRSAV`/`BLOAD` form that
+  accepts packed image data from program text at a stated width and height.
 - **The only vsync is spinning on `TI#`, and the spin is worse than it looks.** Chapter 13
   documents the polling; the demo's first cut had the loop — read `TI#`, branch to yourself
   until it moves — and it did not just waste CPU, it **starved the `PLAY` queue dry**. The