Add the AKLABS megademo example

A six-scene demoscene production that pushes the interpreter to its
edges on purpose: an LCG where there is no RND, redrawn strokes where
the palette cannot be written, the sprite bank as the scrolltext, a
supernova and black hole out of SOLID/COLLISION/RSPPOS, wireframe 3D
from two rotations and a perspective divide, and a single-queue
beeper-style soundtrack with a noise drum break.

The same file runs in the standalone frontend and in the clockless
screenshot host, which gets each scene as a still. Three interpreter
lessons learned the hard way are recorded in the example's README and
TODO.md section 4: a scalar first assigned in a GOSUB dies with its
scope, predeclaring scratch runs the 128-slot variable pool dry, and
a TI# busy-wait starves the PLAY queue of its one release per frame.

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
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

examples/megademo/README.md

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+# AKLABS MEGADEMO '85
+
+A demoscene production for akbasic, written as if it were 1985 and this dialect were
+the machine under the tree. Code and FX by Tachikoma, dedicated to GameCube,
+megagreetz to starfort.
+
+```sh
+./build-akgl/basic examples/megademo/megademo.bas
+```
+
+Q or ESC quits. Six scenes: a title card, a Lissajous braid, a spinning wireframe
+cube and octahedron — which drift together, ignite a pulsing origin point strobing
+the next scene's colours, swallow the camera as the perspective scale outgrows the
+window, and split that origin into the first two raster bars — then the raster
+bars proper, the sprite bank going supernova — the letters fall into the middle, a
+fireball throws them at the walls, and a black hole drags everything back into the
+centre as the transition out — and a finale that composes all of it. A sawtooth
+chiptune in A minor throughout, with a noise drum break, if a SID answers.
+
+## What it leans on
+
+This is a tour of the interpreter's edges, on purpose:
+
+- **No `RND`** — it carries Chapter 17's LCG, seeded from `TI#`.
+- **No palette writes** — the "colour cycle" on the logo is an honest redraw of the
+  same strokes in the next colour, and the raster bars are erased and redrawn two
+  scanlines at a time. The palette is sixteen fixed colours and the demo says so on
+  screen.
+- **Eight sprites** — so the lettering *is* the sprite bank. Each letter of GAMECUBE
+  is drawn with the stroke font from `examples/breakout`, captured with `SSHAPE`,
+  installed with `SPRSAV`, and tinted with `SPRITE`.
+- **`SOLID` walls and `COLLISION 2`** — the supernova registers four rectangles and
+  reverses a bearing per `RCOLLISION(n, 7)`. The interpreter does the physics;
+  BASIC does one subtraction. The letters leave the blast on `MOVSPR`'s continuous
+  form, eight bearings fanned evenly around the compass.
+- **The black hole is `RSPPOS` plus one rotation** — read each letter's position
+  back, rotate the offset from centre, shrink it by 0.86, put it there. Thirty-four
+  frames later everything is in the middle. The screen goes with it: an iris of
+  collapsing black `CIRCLE`s, which works because the drawing verbs clip silently
+  at the window edge — a fireball and an iris can both start bigger than the
+  screen.
+- **The camera is a multiplier** — the wireframe zoom-through is nothing but the
+  perspective scale growing fourteen percent a frame until the solids' edges leave
+  the window, which the silent clipping permits. The pulsing origin point strobes
+  through the bar ramp table because it *is* the next scene: it splits, rides the
+  Y axis to the band edges, and stretches into the first two raster bars in place.
+- **3D with no matrices** — the wireframe scene rotates fourteen pooled vertices
+  through two composed axis rotations and a perspective divide, all of it `SIN`,
+  `COS` and one division, with the float kept on the left of every product because
+  the left operand decides the arithmetic. Erase is the raster bars' trick: redraw
+  the previous frame's twenty-four edges in black.
+- **`PLAY` is one queue** — every note holds it for its length and the whole batch
+  plays on one instrument, so three `V` tracks come out end to end, not layered. The
+  soundtrack is therefore beeper music, the way single-channel tracker tunes were
+  written: sixteenths that take turns being the kick (an `O1` root), the pumping bass
+  (`O2`/`O3` octaves) and the rolling arpeggio (`O4` and up), two alternating
+  eight-beat batches — Am F C G, then Am F Dm E with the G sharp — and every third
+  batch a bar of noise drums. Noise is real: `ENVELOPE`'s sixth argument is a
+  waveform, 0 to 3, which Chapter 7 forgot to mention. Voice 1 carries the tune so
+  the collision blips (voice 2) and scene sweeps (voice 3) never steal its channel.
+
+It also probes for its hardware like a proper boot loader, with one `TRAP` per
+device: no graphics refuses by name and exits, no audio mutes the soundtrack, no
+text grid drops the status row, and a host that never sets the clock — which is
+exactly what `tools/screenshot.c` is — renders each scene as a still and leaves the
+finale as the picture. The same file runs everywhere; nothing hangs.
+
+## The ones that bit
+
+**A scalar first assigned inside a `GOSUB` dies with that call's scope.** The first
+cut computed text centring in a subroutine and read the result after `RETURN`; every
+line of text rendered flush left, because the variable read back as a fresh zero.
+Reads climb the scope chain, so a nested subroutine can *use* your globals — but a
+write to a name that exists nowhere creates it in the current scope, and the scope
+is not coming back. That is why Chapter 18's listings predeclare.
+
+**Then the predeclaring bit back.** The obvious lesson — predeclare everything — ran
+the 128-slot variable pool dry the moment the wireframes arrived: `Maximum runtime
+variables reached`. A predeclared scratch variable occupies a slot forever; a scoped
+one gives its slot back at `RETURN`. So the rule is narrower than it first looks:
+predeclare a subroutine's *outputs* and nothing else. This program predeclares four.
+
+**And the raster wait strangled the SID.** The `VSYNC` label used to spin on `TI#`
+until the jiffy turned — romantic, and fatal. The host releases at most one queued
+`PLAY` note per frame, and a frame of 256 *executed* spin statements is slow enough
+to drop the release rate below the tune's fifteen notes a second. The queue never
+reclaims slots until it drains completely, so the backlog compounded quietly for two
+scenes and then `PLAY queue is full` killed the show. `VSYNC` now sleeps a frame —
+`SLEEP` holds at no per-step cost — and `TUNE` traps the overflow on hosts slower
+than this one, dropping a bar instead of the whole demo. `TODO.md` §4 carries the
+measurement.
+
+## What it wanted and could not have
+
+Recorded properly in `TODO.md` §4, "What a demoscene program wanted and could not
+have": `RND` as a real function, palette writes, `GSHAPE` XOR stamping, a way to ask
+whether `PLAY` has drained, per-voice `PLAY` queues so three voices can actually
+sound together, and a frame-edge wait that is not a busy loop. None of them blocked
+the demo; all of them left a routine behind that every future game will carry too.