feat(parser): respond to OSC 10/11 fg/bg color queries

[roramirez]

What it does

mmterm now responds to OSC 10 (foreground color) and OSC 11 (background color) when a program queries them with ?. The response uses the xterm rgb:RRRR/GGGG/BBBB format with the active theme color.

This unblocks light/dark theme auto-detection in TUIs such as Neovim/Vim (:set background=auto), which query the terminal's background color to pick a colorscheme.

Color set requests (e.g. OSC 10;#ff0000) are ignored on purpose: the PTY must not be able to change the theme.

Changes

• src/terminal/parser.rs: new free function osc_color_query, called from osc_dispatch. Reuses grid.pending_responses (same channel as DSR/DA) and grid.default_fg / grid.default_bg.
• src/terminal/parser_test.rs: 3 tests (fg, bg, and that a set is ignored).
• CHANGELOG.md.

How to test manually

1. In this worktree: cargo run.
2. In the mmterm terminal, run the background query:

   printf '\e]11;?\e\\'; echo

   It should print the response with the theme's background color (e.g. ]11;rgb:1e1e/1e1e/2e2e), without hanging the prompt.
3. Repeat for the foreground: printf '\e]10;?\e\\'; echo.
4. With Neovim installed: open nvim and, with a colorscheme that auto-detects the background, confirm it picks the right variant (or :set background=auto). The result should match the real background (light/dark).
5. Change the mmterm theme and repeat step 2: the reported rgb changes to match the new theme.
6. Regression: a set must not change anything → printf '\e]10;#ff0000\e\\' changes no colors and sends no response.

Tests

cargo test — full suite green (includes osc10_query_reports_foreground, osc11_query_reports_background, osc_color_set_is_ignored).