Speed-independent scope trail persistence (XY plots)

[esaruoho]

Summary

Addresses all of @pfalstad's feedback on this PR. MOSFET lambda part removed — superseded by #313 (MosfetModel class). This PR is now scope-fade-only.

Scope trail persistence — three things you asked for

1. "Vary alpha instead of the number of draw calls" — replaced the alphaCounter > 2 draw-call counter with a real per-frame fade alpha.

2. "Should look the same regardless of simulation speed setting" — the alpha is computed from elapsed wall-clock time, not frame count, using the analytic exponential-decay formula:

   alpha = 1 - exp(-elapsed_ms / trailPersistence)
   

   Same trail at 30fps or 144fps because elapsed time appears in the exponent, not the multiplier.

3. "Still too persistent even at the lowest setting" — the user setting is now a literal time constant in milliseconds (default 200 ms, range 0..2000 ms). Setting it to 0 sets alpha = 1 → instant erase → genuinely no trail at all. So "lowest persistence" actually means no trail.

UX

The Scope Properties dialog now shows a single slider labelled "Trail Persistence (ms)" with a numeric readout next to it (200 ms, 500 ms, …, or none at zero). No more opaque alpha number.

Persistence

Saved per-scope as XML attr tp. Omitted when at the default (200) to keep file size small.

Test plan

• Open an XY scope (e.g. Plot V vs I on a resistor) → trail visible, decays in ~200 ms by default.
• Slide trail persistence to 0 → no trail at all (each frame fully overwrites).
• Slide to 2000 ms → long trail, slow fade.
• Change simulation speed (slow / fast / max) → trail visual decay rate stays the same wall-clock duration.
• Save circuit, reload → tp round-trips for non-default values; default circuits unchanged.

🤖 Generated with Claude Code

[pfalstad]

It still seems too persistent even at the lowest setting. Maybe you should vary alpha instead of the number of draw calls between fade outs?

It would also be nice if it looks the same regardless of the simulation speed setting (as much as practical).

The MOSFET looks fine but I would call it "Lambda" in the dialog (since "Beta" is there) and use "la" as the xml dump attr. You don't need to add it to the StringTokenizer constructor.

[esaruoho]

@pfalstad — major rework based on your feedback. Three changes:

1. MOSFET lambda removed — superseded by Add MosfetModel class with lambda (channel-length modulation) #313 (MosfetModel class), so this PR is now scope-fade-only and the title/description are updated.

2. Alpha varies, not draw-call count — alphaCounter > 2 skip-and-flash is gone. Each frame now fades by an alpha computed for that frame.

3. Speed-independent + lowest setting really is no trail — used the analytic exponential decay:

   alpha = 1 - exp(-elapsed_ms / trailPersistence)
   

   This is what makes it speed-independent: elapsed wall-clock time appears in the exponent, so the same time constant gives the same visual decay rate at any framerate. The user setting is now a literal persistence in milliseconds (default 200 ms, range 0..2000 ms). At 0 the alpha is 1 → instant erase → no trail. The slider readout shows none / 200 ms / etc. instead of a 0..0.1 alpha number.

Force-pushed onto the rebased branch.