feat: ✨ Smooth recorder waveform with frame-interpolated scroll

[lavigarg-simform]

Description

The recorder waveform scrolled in hard, discrete jumps — it shifted one full bar-spacing per new audio sample with nothing drawn between frames. On fast devices (especially Impeller) this looked choppy next to other animated UI.

What changed

• Each pushback now animates the scroll over 80 ms, easing from the current rendered position to the new target instead of snapping.
• The painter keeps two distances: a rendered (animated) one for drawing and a logical (final) one for the overflow check. The check uses the logical target so pushback can't double-fire mid-animation.
• Per-frame repaints during the animation now refresh only the waveform painter, not the whole widget subtree and clipper — keeps the smoothing cheap.
• Clearing the waveform resets all animation state, so no leftover motion carries into the next recording.

RTL recording is unaffected — it advances by index and was already smooth.

No public API change. Internal rendering only.

UI Changes

Recorder waveform scroll is now frame-interpolated instead of stepping in discrete jumps.

Before

Screen.Recording.2026-06-30.at.5.38.05.PM.mov

After

Screen.Recording.2026-06-30.at.5.32.33.PM.mov

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #382