feat(theme-studio): show OKLCH and APCA readouts in the picker

Perceptual-metrics Phase 2. The picker now shows a second readout row under the WCAG ratio: the OKLCH coordinates (L, C, hue°) and the signed APCA Lc against the ground color. WCAG and all three contrast tables are untouched, so APCA stays picker-only for v1.

APCA Lc carries its sign convention in a tooltip and in the README: positive is dark text on a light background, negative is light text on a dark background, so a light color on dupre's dark ground reads negative.

pkReadout drives the new spans from the inlined colormath functions, and a #readouttest headless guard loads dupre-blue and asserts the spans match both the live computation and the known OKLCH reference, with WCAG unchanged.

1 files changed, 7 insertions, 0 deletions

diff --git a/scripts/theme-studio/README.md b/scripts/theme-studio/README.md
index e9817ab1..58ddbf81 100644
--- a/scripts/theme-studio/README.md
+++ b/scripts/theme-studio/README.md
@@ -51,6 +51,13 @@ Three tiers of faces, plus the palette:
   (saturation/value square, hue slider, palette reuse chips, live contrast
   readout, and an any / AA+ / AAA legibility mask). Remove, rename, reorder with
   arrows or drag. The colors serving as background and foreground are locked.
+
+  The picker also shows perceptual readouts beside the WCAG ratio: the OKLCH
+  coordinates (lightness, chroma, hue°) and the APCA Lc contrast against the
+  ground color. APCA Lc is signed — positive means dark text on a light
+  background, negative means light text on a dark background — so a light color
+  on dupre's dark ground reads as a negative Lc. WCAG stays the rating used in
+  the syntax/UI/package tables; APCA and OKLCH are picker-only diagnostics.
 - **Syntax** — every font-lock / tree-sitter category (keyword, string,
   function, type, comment, and the rest), each with normal/bold/italic and a
   contrast rating. Click a category to flash its tokens in the code; click a