Raise themed readable-text mixes to WCAG AA contrast

[fresh3nough]

Summary

Fixes #10533.

Readable text tokens derived as mix(foreground, background, N%) used fixed high blends toward the background. On low-headroom first-party themes that left UI text below WCAG AA 4.5:1:

token	old mix	themes below 4.5:1	worst
pi.json.tpl dimText	52%	19/22	2.17:1
pi.json.tpl mutedText	34%	5/22	3.08:1
claude.json.tpl inactive	40%	10/22	2.72:1
claude.json.tpl inactiveShimmer	25%	2/22	3.70:1
shell.toml.tpl lock placeholder	34%	5/22	3.08:1

These map to actual readable UI (thinkingText, syntaxComment, quotes, lock placeholder), not pure decoration.

Change

token	new mix
dimText / inactive	16% toward background
mutedText / inactiveShimmer / lock placeholder	10% toward background

Hierarchy is preserved (muted stays closer to foreground than dim). Measured after the change: 0/22 themes below 4.5:1, minimum dim contrast 4.55:1 (rose-pine).

Evidence

Reproduced from shipped themes/*/colors.toml only (no hardware):

old dimText @52%   below AA: 19/22  worst=2.17:1
new dimText @16%   below AA:  0/22  min=4.55:1

bash test/shell.d/theme-dimtext-contrast-test.sh
# checked 22 themes x 5 readable-text mixes
# ok - themed readable-text mixes stay at or above WCAG AA 4.5:1

Test plan

• Pre-fix contrast scan across all 22 first-party themes
• Post-fix contrast scan: all readable-text mixes >= 4.5:1
• bash test/shell.d/theme-dimtext-contrast-test.sh
• Optional: omarchy theme set a low-headroom theme (e.g. rose-pine) and spot-check lock placeholder / Pi thinking text