fix(ui-tui): a skin that authors a background owns its polarity

[OutThisLife]

Symptom

Bloomberg demo skin (pure black background, amber text) on light-mode Apple Terminal: agent/markdown body text invisible (black-on-black) while explicitly colored tokens (headers, list markers, status bar) stayed visible.

Root cause

fromSkin resolved polarity with detectLightMode() — the host terminal's polarity — even for skins that author their own background, which the TUI then paints onto the terminal (OSC-11). On light Apple Terminal without truecolor that triggered the light-terminal ANSI normalization pass over a skin whose canvas is black:

• theme.color.text → ansi256(214) — adapted for a light background the skin just painted over
• ansi256(...) fails the OSC-10 hex gate from fix(ui-tui): a skin owns the terminal's DEFAULT foreground (OSC-10) — kills the invisible-text class #69632, so the terminal's default foreground was never repainted → stays the light profile's near-black
• markdown body (rendered with the default fg) = black on skin-painted black

Reproduced exactly:

TERM_PROGRAM=Apple_Terminal (no COLORTERM) + bloomberg skin
detectLightMode: true
text: ansi256(214)   muted: ansi256(245)   ← gray list markers visible, body invisible

Fix

A skin that authors a background owns its polarity. New skinIsLight(): authored-background luminance decides light/dark; host detection only applies to skins without a background (they render on the terminal's own surface). fromSkin also uses the authored background as the reference canvas for the derived tone ladder + contrast adaptation (previously it mixed fills against the cached HOST background — white — for a black-canvas skin). The paired light_colors/dark_colors pick in themeForSkin follows the same rule.

With polarity corrected, text stays truecolor hex, so the OSC-10 default-foreground paint from #69632 engages as designed.

Tests

• dark-background skin on light Apple Terminal keeps truecolor text (no ansi256 bucketing)
• skin background outranks the cached host background for adaptation + tone derivation
• skinIsLight decision table (authored bg decides; host only when absent)

ui-tui: 1347 passed, typecheck + lint clean.

[github-actions]

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CI timings

CI timings · View job

Wall time 7m50s vs 7m28s (+4.9%). 4 job(s) slower, 1 faster, 2 unchanged.

• Build&Test Docker image / build (amd64, ubuntu-latest, linux/amd64, type=gha,scope=docker-amd64, type=gha,mode=max,scope=do...: +38.0s
• Build&Test Docker image / build (arm64, ubuntu-24.04-arm, linux/arm64, type=gha,scope=docker-arm64, type=gha,mode=max,scope...: +12.0s
• OSV scan / Scan lockfiles / osv-scan: -4.0s
• OSV scan / Emit review status: +2.0s
• Check uv.lock / uv lock --check: +1.0s