feat(terminal): add SGR conceal (attribute 8, reset 28)

[roramirez]

Summary

Implements the SGR conceal attribute (ESC[8m, reset ESC[28m): concealed text is rendered in the background color so it is invisible. Used by password prompts and some TUIs. Follows the exact pattern of the existing SGR attributes (bold/dim/underline/…): tracked on the pen, stamped per cell, reset by SGR 0/28, and preserved across DECSC/DECRC and the alternate screen.

Changes

• src/terminal/grid.rs: add conceal 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 arms 8 (conceal on) and 28 (conceal off). Also collapse the DECSET/DECRST (h/l) arms of handle_dec_private_modes into ('h' | 'l', …) patterns (same behavior, fewer lines).
• src/renderer/draw_fns.rs: in resolve_cell_colors, paint the glyph in the background color when cell.conceal — this also hides decorations, which derive from the resolved fg.
• Tests: parser tests for stamping/reset; a resolve_cell_colors unit test in text_test.rs.
• CHANGELOG.md: entry under [Unreleased] / Added.

How to test

• cargo test (see sgr_conceal_* in parser_test.rs and resolve_cell_colors_conceal_paints_fg_as_bg in text_test.rs).
• In a running mmterm session: printf '\033[8mSECRET\033[0m\n' shows nothing (text painted in the background color); selecting it still copies SECRET.

Checks

• cargo fmt
• cargo clippy --locked -- -D warnings
• cargo test (1211 passed)
• [~] km score --trend origin/main --fail-if-worse — grade B+ (passes .kimun.toml fail_below = B); trend -0.02 on File Size. The codebase stores the SGR pen redundantly across Cell/Grid/SavedCursor/SavedScreen, so adding one attribute consistently costs ~13 lines in grid.rs; offset as far as reasonable by simplifying the DEC private-mode setter. Keeping conceal in the Saved* structs is deliberate — dropping them to gain the 0.02 would make conceal the only attribute not preserved across DECSC/DECRC.
• CHANGELOG.md updated