feat(pool): the current — a drag stirs a slow, persistent flow that keeps carrying the light

[pyramation]

Summary

Gestures now deposit motion, not just colour. Under the Pool's light sources sits a coarse velocity grid — the current. Dragging pushes on it; after release the blobs are carried by it. Pull a spot and it stretches, circle and the pool keeps turning slowly for minutes, then comes to rest. This is the "pulling clay that then animates itself" feel.

@wavegrid/pool — new Current (packages/pool/src/current.ts)

16×16 grid of (u,v) over the 0..1 square
push(x, y, vx, vy, radius, dt, maxSpeed)   // Gaussian push from a finger, speed capped (≤0.08 units/s)
step(dt)                                   // diffuse → no-flow walls → pressure projection → exp decay (τ=90s)
at(x, y)                                   // bilinear sample
release()                                  // τ → 2.5s (Dissolve)
reset()                                    // still at once (Blackout / other producer takes the grid)
snapshot()                                 // [] when still, else 512 rounded numbers

The pressure projection (a few Gauss–Seidel sweeps) is what makes a push turn into a swirl instead of piling everything in a corner: the water is incompressible, so pulling one side brings the other round.

PoolField changes

• Every smoothed touch calls current.push(...) with its velocity; max speed scales with Motion (lerp(0.3, 1, motion) * CURRENT_MAX_SPEED).
• Blob sources are advected: s.x += (s.vx + current.vx) * dt (rings keep their own ripple path).
• release() / reset() forward to the current; snapshot() gains current: number[].
• Works in Fade and Hold; in Hold this is where it shines — held colour is literally pulled around.

UI

• socket-state parses current (defaults to []).
• PoolCanvas draws faint white streaks per cell where the water moves — hidden in Beams only, so that view still shows exactly what the lasers get.

No new controls, no protocol changes: server still emits ordinary HSB paint/cannon cells via the same smoother; Clear/Stop/animation precedence unchanged (server tests still 100/100).

Tests (packages/pool)

• Circling drag → measurable swirl after release, slow (< 0.1 u/s), blobs carried a fraction of a lap in 20 s, stays inside the pool.
• Unattended current decays over tens of seconds; release() stills it within seconds; reset() at once; still pool sends current: [].

Link to Devin session: https://app.devin.ai/sessions/c4872c2982734093bdee80d04a7d5ceb
Open in Devin Desktop: https://app.devin.ai/desktop/session/c4872c2982734093bdee80d04a7d5ceb?variant=devin
Requested by: @pyramation

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[tenki-reviewer]

Review complete. 🟡 1 medium

💬 Inline comments (1)

• 🟡 Resend full current grid on every pool frame — pool-field.ts:466

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This PR introduces a flow/current simulation to the pool output: current.ts computes an incompressible velocity field over the grid, pool-field.ts integrates it into PoolSnapshot and the pool state machine, and the UI (pool-canvas.tsx, pool-tab.tsx, socket-state.ts) projects and renders the velocity streaks. It also adds unit coverage in pool-field.test.ts.

Files	Change
packages/pool/src/current.ts, pool-field.ts, index.ts	Add the fluid current velocity grid to the pool model and expose it via the snapshot.
packages/pool/__tests__/pool-field.test.ts	Unit tests for the new state-machine current integration.
packages/ui/src/lib/socket-state.ts, pool-tab.tsx, pool-canvas.tsx	Receive and render the current grid as animated streaks in the pool canvas.

One medium finding: the full 512-element current grid is re-serialized into every ~15 Hz pool frame even though painted cells are sent only on change, adding per-client wire cost.

_{Reviewed commit: b885847}

[tenki-reviewer]

Adds a fluid-velocity 'current' field to the pool snapshot and renders its streaks in the UI canvas, synced over websockets.

Key findings

• 🟡 Resend full current grid on every pool frame — pool-field.ts:466