fix(tui): paint the OSC-10 default foreground on quantizing terminals

[OutThisLife]

A skin that authors a background paints both terminal defaults: OSC-11 for the backdrop, OSC-10 to re-base every default-fg token — markdown body, borders, anything rendered without an explicit color — onto the theme's text tone.

The OSC-10 half never fired on a limited-palette terminal. normalizeThemeForAnsiLightTerminal rewrites the foreground tones to ansi256(N), and setTerminalForeground only accepts #rrggbb, so the argument failed the hex test and the write was silently skipped. The background moved to the skin while default-fg text stayed on the host profile's foreground.

That split is the reported symptom. Prose renders in the terminal's own near-black while every themed token beside it renders the skin's gray, so the base text color appears to change between adjacent words — and because a resize repaints affected cells from the screen buffer, the text "goes black" on resize and the mix reads as scattered rather than uniform.

This resolves the tone through a new themeToneHex before handing it to OSC-10: ansi256(N) maps through the xterm grayscale ramp and 6×6×6 cube, an authored hex passes through, and a tone with no paintable color yields '', which correctly clears back to the terminal's own default. That keeps the existing opt-in intact — a skin without a background still releases both defaults.

Verification

On Terminal.app with the brooklyn skin, theme.color.text resolves to ansi256(238). Driving the real paint path through a fake TTY and capturing the bytes:

before:  ESC]11;#f6f9fd BEL                        <- background only
after:   ESC]11;#f6f9fd BEL  ESC]10;#444444 BEL    <- both defaults

Regression tests are invariant-shaped rather than snapshots: the palette-wide case asserts that every tone in a quantized theme resolves to a literal #rrggbb, so a newly added tone can't silently regress the paint. Reverting the source fix while keeping the tests turns them red (4 failed), which confirms they actually guard the behavior.

tsc --noEmit is clean. The full ui-tui suite has 24 pre-existing failures on origin/main (subscription overlay, syntax highlighter, backpressure, paired-palette); the failure set is byte-identical before and after this change, so it introduces none.

[github-actions]

૮ >ﻌ< ა ci review

_{ran on 0b1ee22}

all good!