A volume slider with an underwater world

Make "Deep Dive Switch", No. 063 in a series of web material studies: a depth switch that goes from light to dark, from the sea's surface to the floor of the abyss. Build it as one self-contained page with no libraries: raw WebGL2, Web Audio and a 2D pixel buffer.

PAGE
- Editorial layout: a small kicker "Material Studies / No. 063 · WebGL + Web Audio", an italic serif title "Deep Dive Switch.", and a one-line caption.
- The switch sits centred and large. Below it go three readouts (Depth, Zone, Music), a hint line and a "Sound on/off" button.
- The whole page's colours (background, text, lines) follow the depth: pale aqua at the surface, near-black blue in the abyss.
- It must work at phone width.

THE SWITCH
- A pill-shaped switch in a moulded bezel: white glossy plastic at the surface, deep navy in the abyss.
  - A soft drop shadow falls on the page and fades to nothing before the canvas edge.
  - A slight inner shadow runs along the top of the track.
- Inside it is a 176×50 pixel-art cross-section of the sea.
  - Redraw it every frame on the CPU and show it with nearest sampling, never smoothed.
  - Use whole device pixels per scene pixel where there is room.
- The knob is a glass bathysphere: a vertical capsule of clear glass, a little taller than the track, drawn in a single full-canvas WebGL2 fragment shader.
  - It magnifies what is under it (about 1.28×) and bends it.
  - At its rim it splits R, G and B. A darker refraction band sits inside a bright edge, with a Fresnel tint and a faint aqua cast.
  - Two highlights: a long soft streak at the upper left and a small dot at the lower right.
  - Light gathers toward its foot, and it casts a soft shadow on the scene.
- Motion:
  - It follows the finger on a spring, lagging a little.
  - It stretches sideways with speed and squashes vertically to keep its volume.
  - It wobbles on a damped spring driven by its acceleration.
- Controls:
  - Drag the knob, or drag anywhere on the track.
  - Tap to send it to the other end; it travels there on its spring.
  - Arrow keys, Home/End and Space/Enter also work.
  - The switch has role="slider" with ARIA values.

THE SEA (left = shallow, right = deep; an original palette)
- Sprites are filled shapes stamped with an automatic 1-px outline and a light top / dark bottom.
- The sea floor, left to right:
  - a sandy beach with a sandcastle;
  - a reef plateau with rocks;
  - a drop-off into rocky deep water;
  - the abyssal floor rising again at the far right.
- Up top on the left is sky with a sun, two flapping gulls and a little red sailboat bobbing on animated waves with a foam line.
  - The surface is level over the shallows, then rises out of the picture toward the deep, so the view seems to sink.
- The shallows:
  - turquoise water with swaying slanted god rays and moving caustics on the sand;
  - branching corals in pink, orange and violet, brain corals, swaying sea grass;
  - an anemone with a little orange-and-white fish darting about it;
  - a sea turtle gliding back and forth, paddling;
  - a school of yellow fish circling over the reef that scatters away from the knob.
- The twilight zone:
  - the water darkens to deep blue;
  - three translucent jellyfish, pink and violet, pulse and trail wavy tentacles; they glow softly and pulse faster when the knob comes near;
  - every half minute a faint whale silhouette swims past in the background.
- The midnight zone:
  - a long swaying chain of glowing cyan-and-green beads (a siphonophore);
  - small dark lanternfish with blinking light spots;
  - marine snow drifting down.
- The abyssal floor:
  - a hydrothermal vent breathing bubbles;
  - an anglerfish with a rod and a glowing lure;
  - a treasure chest.

LIGHT AND THE DIVE
- Daylight falls off with depth and dims a little toward the floor. It is quantised and Bayer-dithered so the gradient steps like pixel art.
- Below the shallows the bathysphere's searchlight comes on: a stepped, dithered pool of light around the knob, with a blue haze scattered in the water.
- Inside the lens everything is shown lit.
- Behind the knob trail rising bubbles and, in the dark, sparks of blue bioluminescence.
- Glowing crescent echoes trail behind it: pale aqua in the shallows, cyan in the deep.
- The lure lights its own little pool.
- As the knob nears, the anglerfish turns and opens its toothy mouth wide, and its lure blazes.
- At the very bottom the chest lid swings open on glowing gold, and gold glints fly up from it.

SOUND — all synthesised live with Web Audio, original music
- 84 BPM, a four-bar loop in D minor (Dm7 · B♭maj7 · Gm7 · A7sus4), on one lookahead-scheduled clock with a little swing.
- Layers fade in and out with depth:
  0. The surface: waves (filtered noise swelling slowly) and gull cries. The moment you go under, a splash, and the waves drop to a dull underwater roar.
  1. The reef: a kalimba ostinato (sine with bright, quickly fading overtones, in a long reverb), with bubbles popping on the off-beats.
  2. A soft dub beat: a round kick, rim clicks into a dotted echo, a swung shaker, a deep bass.
  3. The twilight: slow detuned pads, a whale singing now and then (a gliding, vibrato tone through a formant filter), then sonar pings ringing in the echo.
  4. The abyss: a breathing sub drone, a slow lub-dub heartbeat, and glassy arpeggios of high pentatonic notes ringing in the echo.
- The music sits behind a gentle low-pass that closes a little with depth, about 5 dB under the switch's own sounds.
- Switch sounds:
  - a bubble bloop when grabbed, a plop when released;
  - a water swish and bubble pops that grow with drag speed;
  - a splash crossing the surface either way;
  - a glass chime past each jellyfish;
  - the angler's deep gulp;
  - a fanfare when the chest opens: a rising kalimba arpeggio, a low boom and a shimmer.
- Audio starts on the first touch; a Sound on/off button.

READOUTS
- Depth, 0 → 6,000 m, mapped to the zones.
- Zone: Surface / Sunlit / Twilight / Midnight / Abyss.
- Music: Waves / Kalimba / Reef dub / Whale song / Sonar / Abyss.