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/nord.json

@@ -10,7 +10,7 @@
     "surface": "#3b4252",
     "overlay": "#4c566a",
     "text": "#d8dee9",
-    "subtext": "#e5e9f0",
+    "subtext": "#b7becc",
     "muted": "#616e88",
     "accent": "#81a1c1",
     "accentAlt": "#b48ead",