feat(pool): Hold works in Droplets — each ripple leaves its colour behind

[pyramation]

Summary

Follow-up to #128, addressing the reviewer finding that Hold was inert in Droplets mode: rings always grow, thin and get culled at ring >= 1.6 regardless of hold, so the toggle promised something it didn't do.

Rather than hiding the toggle, Hold now means something in Droplets: a droplet is a ring that ripples out and, when holding, a puddle that stays behind.

private drop(x, y, color) {
  this.deposit(x, y, 0, 0, color, 'ring');
  if (this.settings.hold === true) this.deposit(x, y, 0, 0, color, 'blob');
}

Both droplet deposit sites (pointerDown, drag beats in step) go through drop(). The blob goes through the existing hold path, so re-dropping on the same spot recolours the puddle instead of stacking sources. Fade mode is unchanged (no blob).

UI: the Feel hint reads "Ripples still travel out, but each leaves its colour behind." when Hold + Droplets.

Test: hold + droplets keeps the centre cannon lit in the touch colour 40s later with exactly one source left; fade + droplets goes to black.

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

[devin-ai-integration]

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[tenki-reviewer]

Review complete. No issues found — approved ✅.

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This PR reworks how the light-field pool deposits a source: a new drop() path creates a ripple ring at the touch and, in hold mode, adds a blob, while deposit() gains position-blended sampling and energy-aware MAX_SOURCES eviction. The UI pool-tab.tsx is refactored for mode/hold interaction, and coverage is added in pool-field.test.ts.

Files	Change
packages/pool/src/pool-field.ts	Introduces ring/blob drop(), position-blended deposit(), tightened source eviction, and updated sampleAt() energy summation for held strokes.
packages/pool/__tests__/pool-field.test.ts	Adds unit tests for the new hold/droplet deposition and eviction behavior.
packages/ui/src/components/pool-tab.tsx	Refactors mode/hold control wiring and help text to match the simulator's ring/blob semantics.

Note: the sweep sub-scan did not publish; adjudication relied on the other reviewers. Two candidate concerns (held-droplet double energy stackup at the touch and ring eviction under the 240-source cap) were verified but judged below the flagging confidence threshold, so this review closes as approve.

_{Reviewed commit: 676cd46}