fix(themes): P1 palette retunes from the design audit + gated subtext floor (item 28b)

The audit's P1s, fixed at the palette level (items 25/27 previously
patched the consumers around these):
- summer-day: surface was the TEXT color (slate #5c6a72 on cream) and
  subtext was the BASE (ratio 1.0, invisible) — now a raised cream
  surface #efe7d0 and a slate-teal subtext #6e828a in the theme's
  family.
- flexoki-light: surface/overlay/muted were all the text black and
  subtext the paper — retuned to canonical upstream Flexoki tones
  (ui #E6E4D9, #B7B5AC, tx-2 #6F6E69, base-500 #878580).
- miasma: pure-black surface under a #222 base → raised #2e2e2e.
- kanagawa: surface was sumiInk0 (darker than the bg) → upstream's
  raised sumiInk4 #2a2a37.
check-theme-contrast.py gains the adopted rule: subtext on base >= 3.0
(all 21 themes pass), so the invisible-secondary-text class cannot
ship again. Model note in the backlog: `overlay` is a mid-tone role
(catppuccin semantics), not a third bg tier.

Verified: V0 + checks.theme-contrast green (21x3); re-audit shows all
four P1 findings gone; a scratch downstream with summer-day active
builds and the new surface hex lands in the rendered ghostty config.
Remains: V3 four-theme eyeball (HARDWARE-QUEUE) and 28b's P2 floors +
exemption list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

themes/flexoki-light.json

@@ -7,11 +7,11 @@
   "colors": {
     "base": "#FFFCF0",
     "mantle": "#d9d6cc",
-    "surface": "#100F0F",
-    "overlay": "#100F0F",
+    "surface": "#E6E4D9",
+    "overlay": "#B7B5AC",
     "text": "#100F0F",
-    "subtext": "#FFFCF0",
-    "muted": "#100F0F",
+    "subtext": "#6F6E69",
+    "muted": "#878580",
     "accent": "#205EA6",
     "accentAlt": "#CE5D97",
     "good": "#879A39",