bauhaus: fix scroll step calculation for large-range sliders

[kadykov]

Problem

The dt_bauhaus_slider_get_step() function computes how much a slider value
changes per scroll-wheel tick. Since the February 2022 refactor
(commit f7561c5e79), the function contains this branch:

if(top >= 100)
    step = 1.f;

For any slider whose visible span (soft range) is ≥ 100, the step is always
hardcoded to 1 — bypassing the logarithmic formula that produces a proportional,
round-number step.

This is unnoticeable for sliders where "1 unit" is a natural granularity (e.g.
a 0–100 % slider, a 0–360° hue slider). However it produces unusable behaviour
for sliders measured in large-valued units. The clearest example is the
color temperature parameter in the color calibration module
(soft range 3000–7000 K, span = 4000):

• Actual: 1 scroll tick = 1 K — 100 ticks needed to change by 100 K, barely perceptible.
• Expected: a proportional step, comparable to how the sigmoid contrast
  slider (soft range ~0.7–3.0, span ~2.3) scrolls at 0.01 per tick (~0.4 % of span).

Root cause

The log formula correctly produces "~1 % of visible span, rounded to the nearest
1/2/5 × 10ⁿ". It was always intended to cover all ranges; the >= 100 guard
is an unnecessary short-circuit that happens to give the same answer as the
formula for spans 100–315, but diverges above that.

Fix

Remove the >= 100 guard. The formula now runs for all ranges:

// before
const float top = fminf(max-min, fmaxf(fabsf(min), fabsf(max)));
if(top >= 100)
{
    step = 1.f;
}
else
{
    step = top * fabsf(d->factor) / 100;
    // … log rounding …
}

// after
const float top = fminf(max-min, fmaxf(fabsf(min), fabsf(max)));
step = top * fabsf(d->factor) / 100;
// … log rounding …

Impact

Soft-range span	Step (before)	Step (after)	Example
100 – 315	1	1 (no change)	percentage 0-100 %, hue −180…+180°
316 – 794	1	5	0-360° hue (colorbalancergb), radius 0-500 px
795 – 3162	1	10	sigmoid white target 20-1600 nits
3163 +	1	50 / 100 / …	temperature soft 3000-7000 K → 50 K ✓

Modules that need a non-formula step can still call dt_bauhaus_slider_set_step()
to override (as contrastntexture.c already does for a very fine 0.0001 step).

Fix #21855

Testing

• Color calibration temperature slider: scroll wheel now moves in 50 K increments
  (soft range 3000–7000 K active), comparable to Lightroom's default WB step.
• Old white balance module temperature slider (hard range 1901–25000 K, no soft
  range): scroll moves in 100 K increments.
• Sigmoid contrast slider (soft range 0.7–3.0): step unchanged at 0.01 ✓
• Exposure compensation (range ±18 EV, span ≤ 36 < 100): step unchanged ✓
• 0–100 % sliders: step unchanged at 1 ✓

[Copilot]

Pull request overview

This PR fixes mouse scroll-wheel step sizing for Bauhaus sliders with large visible (soft) ranges by removing a hard-coded step = 1 shortcut and always using the existing logarithmic “~1% of range” rounding logic. This improves usability for large-unit sliders like color temperature (Kelvin) while keeping behavior unchanged for common small-range sliders.

Changes:

• Remove the top >= 100 guard that forced step = 1.f for large soft ranges.
• Always compute the scroll step from the proportional/log-rounded formula for all ranges.

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