perf(effects): snap fade_out/fadeToBlackBy to target on final step

[aenertia]

fade_out() and fadeToBlackBy() use integer multiply-divide which never reaches exactly zero — pixels linger at 1-2 brightness counts indefinitely. This prevents clean transitions and wastes CPU on effects that should have faded to black.

Fix: when the result would be ≤ the snap threshold (typically 1-2 counts), snap directly to the target value. Eliminates the long tail on fade transitions and makes Copy Segment / DDP realtime transitions crisp.

Upstream issue: #4976

Summary by CodeRabbit

• Bug Fixes
  • Improved fading behavior so channels reach their target values accurately, including when no gradual change is calculated.
  • Enhanced fade-to-black effects to ensure pixels visibly darken instead of remaining unchanged due to rounding.

[coderabbitai]

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• wled00/FX_fcn.cpp

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Walkthrough

The fade functions now handle zero-progress scaling explicitly. fade_out assigns the target channel value, and fadeToBlackBy stores its scaling factor locally before applying it.

Changes

Fade behavior

Layer / File(s)	Summary
Fade calculation updates wled00/FX_fcn.cpp	fade_out snaps channels to their targets when scaling produces no progress. fadeToBlackBy retains its early returns and stores 255 - fadeBy in scale before scaling each pixel.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: 🟡 Moderate · up to 17bc6

The fadeToBlackBy transition may still leave pixels lingering instead of reaching the target, so the claimed fade-to-black fix is incomplete. Merge should wait for this correctness issue to be resolved or explicitly accepted.

Suggested labels: bug

Suggested reviewers: dedehai, softhack007

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: snapping fade_out and fadeToBlackBy values to the target on the final step.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[coderabbitai]

Actionable comments posted: 2

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Inline comments:
In `@wled00/FX_fcn.cpp`:
- Line 1077: Update the comment immediately preceding the delta check in the
relevant effect function to describe the actual snap behavior: when integer
scaling yields delta == 0, c1 is set directly to c2, making the channel snap to
its target. Do not change the code behavior.
- Around line 1095-1103: Update fadeToBlackBy’s scaling logic so the
low-brightness snap behavior is implemented with an explicit threshold that can
actually be reached, or remove the unreachable scaled == c condition if snapping
is not required. Preserve the existing inactive and fadeBy == 0 early returns
and per-pixel scaling flow.

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• wled00/FX_fcn.cpp

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[DedeHai]

I'd have to say no to this. it changes behaviour.

[aenertia]

Fair point — the fade_out snap does change the visual behavior at the tail end of fades (motivated by wanting crisp transition for DDP under constrained resource conditions). Happy to close this one. Thanks for the review.