Feat: Add soft-clip tone mapping with legacy Reinhard

Replace the per-channel Reinhard display transform with a parameterized
soft clip T_p(x) = tanh(x^p)^(1/p), default softclip p=2, exposed through
--tone-map and --tone-map-p. Keep --tone-map reinhard bit-compatible with
the previous x/(1+x) curve for existing images and reject combining it
with an explicit --tone-map-p.

Route the primary image and the mesh overlay through the same
ToneMapSettings; the linear HDR FITS writer stays pre-tone-map. Add a
focused optics-linked tone-map test target, CLI success/error coverage,
and document the display operator versus the Moffat effective PSF.

usage.md

@@ -267,6 +267,46 @@ at the existing cache radius. Images above `F` retain the direct fallback.
 `--max-magnification M` (default unlimited) caps the per-triangle rendering
 magnification before flux is formed; it is an explicit preview approximation.
 
+### Tone mapping and display
+
+`--exposure` is a linear multiplier applied while the HDR framebuffer and its
+PSF splats are accumulated. The Moffat profile is the empirical effective
+optical PSF, absorbing seeing, jitter, and residual aberrations; it is not
+changed by any display choice.
+
+The tone map is a later camera/display response applied to the finished linear
+HDR image, so it does not alter the linear HDR PSF. The default operator is a
+parameterized, per-channel soft clip
+
+\[
+T_p(x) = \left[\tanh(x^p)\right]^{1/p}, \qquad p \ge 1,
+\]
+
+selected with `--tone-map softclip --tone-map-p 2`, i.e.
+
+\[
+T_2(x) = \sqrt{\tanh(x^2)}.
+\]
+
+It is close to the identity in the dark, then rolls smoothly into a saturated
+shoulder above \(x \sim 1\). Because it is applied independently to each linear
+RGB channel, a bright star transitions from its colored Moffat wings into a
+white saturated core. Larger `p` approaches \(\min(x,1)\). This per-channel
+response is what gives the current simplified preview pipeline the crisp white
+cores seen in real astrophotography, rather than the gray, soft cores a
+Reinhard curve leaves on bright stars.
+
+`--tone-map reinhard` selects the historical \(x/(1+x)\) display transform and
+is retained only to reproduce output rendered before the soft-clip operator was
+introduced. FITS output always remains the pre-tone-map linear RGB framebuffer,
+independent of operator and `p`; use it to measure physical flux, peak values,
+and FWHM. Tone-mapped PNG/PPM data are display-referred and must not be used for
+those measurements.
+
+Commands written before this change that omit `--tone-map` actually used
+Reinhard; add `--tone-map reinhard` explicitly to reproduce their PNG/PPM
+output. Historical FITS/HDR benchmarks are unaffected.
+
 ### Fast preview mode
 
 `--fast-mode` replaces the per-event PSF splat with a two-stage approximation:
@@ -372,7 +412,8 @@ ordinary output, replacing its extension with `_HDR.fits`; for example,
 `output/imgs/ring_HDR.fits`.  It writes the pre-tone-mapping RGB
 framebuffer as a three-plane, 32-bit float FITS image. Values remain linear HDR
 at the renderer's arbitrary scale; no tone mapping or per-frame normalization
-is applied. The ordinary
+is applied, and the `--tone-map` operator and `--tone-map-p` value never affect
+this file. The ordinary
 binaries do not contain this option or writer.
 
 The FITS header describes a synthetic 8640-by-5760, 36-by-24 mm full-frame