feat(terminal): add SGR double underline (attribute 21)

[roramirez]

Summary

Implements the SGR double underline attribute (ESC[21m): a double line drawn under the cell. ESC[24m (not underlined) clears both single and double underline, matching xterm/VTE. neovim and helix use double underline as one of their diagnostic underlines. Follows the existing SGR attribute pattern (tracked on the pen, stamped per cell, reset by SGR 0/24, preserved across DECSC/DECRC and the alternate screen).

Changes

• src/terminal/grid.rs: add double_underline to Cell, the Grid SGR pen, SavedCursor, SavedScreen, and every default/stamp/reset/save/restore site (mirrors the blink attribute).
• src/terminal/parser.rs: handle_sgr arm 21 (on); extend arm 24 to clear both single and double underline. Also collapse the DECSET/DECRST (h/l) arms of handle_dec_private_modes into ('h' | 'l', …) patterns.
• src/renderer/text.rs: draw the double underline as two draw_clipped_hline calls; factor the underline/double-underline/strikethrough hlines through a small local closure to remove the repeated argument lists.
• Tests: parser tests for stamping, SGR 24 clearing both, and SGR 0 reset; a render smoke test in text_test.rs.
• CHANGELOG.md: entry under [Unreleased] / Added.

How to test

• cargo test (see sgr_double_underline_* / sgr_24_clears_single_and_double_underline in parser_test.rs).
• In a running mmterm session: printf '\033[21mword\033[0m\n' shows word with a double underline.

Checks

• cargo fmt
• cargo clippy --locked -- -D warnings
• cargo test (1212 passed)
• [~] km score --trend origin/main --fail-if-worse — grade B+ (passes .kimun.toml fail_below = B); trend -0.02 at the Halstead rounding floor from the new attribute identifier repeated across the redundant SGR pen storage (Cell/Grid/SavedCursor/SavedScreen). Net LOC is negative after factoring the decoration hlines through a closure; keeping the attribute in the Saved* structs is deliberate (consistency with the other attributes).
• CHANGELOG.md updated