Click effects land off the drawn cursor under cursor smoothing #1335

Problem

With the default cursor smoothing (0.67), the flat cursor sprite trails the recorded pointer
by about 190 ms. Every click effect is anchored to the raw click position, so when a click
follows a move closely, the effects land beside the cursor:

• the impact ring and the click shader effects (ctx.clicks) sit on the raw point;
• ctx.cursor, given to the same effects, sits on the lagging smoothed point, so an effect
  reading both sees them disagree;
• auto zooms frame the raw click while the cursor arrives late.

It shows most on scripted takes (eased move, click about 100 ms after the stop) and on fast
human clicks. A human who dwells before clicking hides it: the spring has caught up.

Root cause

crates/compositor/src/frame_geometry.rs, CursorPlacer::at:

fn at(&self, t: f32) -> Option<(f32, f32)> {
    if self.model3d { self.track.pinned_at(t) } else { self.track.at(t) }
}

The 3D cursor already converges on the click (CursorTrack::pinned_at: eases onto the raw click
point over 250 ms, holds it 100 ms, releases over 250 ms). The flat sprite, the default, skips
it and draws track.at(t), the smoothed spring output.

Measurement

Exact port of CursorTrack::smoothed() (240 Hz spring) on two real sidecars, 1920×1080 output.
Distance between the drawn cursor and the click point, at the click.

Demo take (Playwright, eased move, 2 clicks)

Human take (WGC capture, dwell before each click): 0 px at click for every value.

Also noted: the curve jumps at 0 (passthrough, then 45 ms of lag at 0.01), and the slider's
upper half only covers a third of the spring's extended range.

Proposed fix

1. Pin the flat sprite too. CursorPlacer::at always returns pinned_at(t). The drawn
   cursor, ctx.cursor, ctx.clicks, the bounce and the impact ring then meet at the click,
   at any smoothing value. One line, both renderers share it, and pinned_at already has its
   test (pinned_at lands on the raw point at 0, 0.5 and 1).
2. Close clicks each keep the tip. pinned_at starts a click's approach only after the
   previous click's hold: with clicks 200 ms apart, the next approach pulled the tip off the
   first click at its own instant.
3. Lower the default from 0.67 to 0.3 (src/lib/projectDefaults.ts), for new projects
   only: about 95 ms of lag, still removes capture tremor. Saved projects keep their value.

Not in scope: the auto-follow camera keeps its own damping (follow_at), by design.