fix(themes): fidelity hierarchy nits + #52 residual ANSI (#67, #68)

#67 hierarchy: nord subtext dimmer than text; tokyo-night btop Night
blues; ethereal surface raise + muted darker than overlay; retro-82
surface raised; ristretto subtext distinct + btop main_bg align;
flexoki-light distinct bright-black; gruvbox overlay/subtext ladder
(no rename).

#68 residual ANSI: vantablack ansi[8]=muted; osaka-jade ansi[3]=warn.

Verified: python3 tools/check-theme-contrast.py themes/ → 24×7 all pass.

themes/gruvbox.json

@@ -8,9 +8,9 @@
     "base": "#282828",
     "mantle": "#222222",
     "surface": "#3c3836",
-    "overlay": "#3c3836",
+    "overlay": "#504945",
     "text": "#d4be98",
-    "subtext": "#d4be98",
+    "subtext": "#a89984",
     "muted": "#7c6f64",
     "accent": "#7daea3",
     "accentAlt": "#d3869b",