0.39.0 — One unified palette model (no gray "mode")

Changed (breaking)

• The chromatic/gray palette split collapses into a single OKLCH (Lightness /
  Chroma / Hue) model. A neutral is no longer a special "mode"; it is an
  ordinary low-chroma palette with calm defaults (a low but non-zero base chroma,
  a wider neutral lightness ramp). One derivation path (computePaletteColor)
  now serves every palette, and the Neutral / Alternate editors show the same
  full L/C/H picker, lock-to-500 toggle, and derived-scale snapping as accents.
  PaletteConfig drops its gray vocabulary (tintHue, tintChroma,
  grayLightnessCurve, graySaturationCurve); baseColor, lightnessCurve,
  and saturationCurve are now universal, and the internal mode prop is
  removed from the palette editors.
• Token names are unchanged — this is a theme-config schema change, not a
  tokens.css migration, so no consumer CSS or token references are affected.
  The shipped default.json was regenerated; the neutral shift is sub-1-LSB per
  channel and every derived --surface-* / --text-* / --color-* token is
  byte-identical.

Fixed

• A component's surface control no longer vanishes when its fill is a flat
  colour. componentGradientSource returned undefined for any non-gradient
  alias and GradientEditor renders only {#if gradient}, so the surface editor
  disappeared whenever a fill was a plain colour (e.g. SectionDivider's default
  transparent). The editor now synthesizes a none/solid single-stop snapshot from
  a flat (or absent) alias so the type picker always renders, and promotes the
  flat alias to a real gradient on first edit. SectionDivider's section heading is
  renamed "Background" → "Surface" to match Panel and disambiguate it from the
  preview backdrop control.
• Palette colour overrides now apply live while you drag. A new Text /
  Surfaces / Borders override previously reached neither the live page nor the
  preview swatch until commit — handleColorChange only wrote to the store when
  the key was already an override. Every drag tick now writes (the open session
  collapses to one undo entry; cancel/confirm clean up no-ops), and the per-step
  hex text tracks the live colour too.

Migration

• Consumer themes migrate automatically on load. unifyGrayPalettes (run in
  loadFromFile after renamePrimaryPaletteKey) close-maps existing neutrals to
  the unified form: baseColor snaps to the effective step-500 colour (preserving
  the subtle tint and the neutral lightness ramp), the saturation curve becomes
  flat-100, and the palette is locked to base. Every palette drops the four
  vestigial gray fields, and the gray-lightness / gray-saturation curve-offset
  keys fold into lightness / saturation. Default and flat-saturation neutrals
  are visually identical; only a hand-shaped gray saturation curve migrates
  approximately and may want a quick manual retune.