feat(terminal): add DECRQM (CSI ? Ps $ p) — report DEC private mode state

[roramirez]

Summary

Implements DECRQM (CSI ? Ps $ p), the Request Mode query for DEC private modes. mmterm now replies CSI ? Ps ; Pm $ y where Pm is 1 (set), 2 (reset), or 0 (mode not recognized). Applications such as neovim use DECRQM to probe terminal capabilities before enabling a mode, avoiding redundant or unsupported mode switches.

Changes

• src/terminal/parser.rs:
  • Route every ?-prefixed CSI through handle_dec_private_modes (vte reports the $ intermediate alongside the ?, so DECRQM does not collide with plain set/reset).
  • Handle action == 'p' as DECRQM: reply from a new decrqm_state helper that maps a mode number to 0/1/2 over the state mmterm already tracks (?1, ?7, ?25, ?1000/1002/1003, ?1004, ?1006, ?1049, ?2004).
  • Collapse the DECSET/DECRST (h/l) arms into ('h' | 'l', …) patterns using a single on = action == 'h' flag — same behavior, fewer lines.
• src/terminal/parser_test.rs: a test covering set, reset, reset-after-disable, unknown-mode, and that a query does not toggle the mode.
• CHANGELOG.md: entry under [Unreleased] / Added.

How to test

• cargo test (see decrqm_reports_mode_state in parser_test.rs; the full suite still passes with the refactored setter).
• In a running mmterm session: printf '\033[?2004$p' replies \033[?2004;1$y when bracketed paste is enabled.

Checks

• cargo fmt
• cargo clippy --locked -- -D warnings
• cargo test (1204 passed)
• km score --trend origin/main --fail-if-worse (PASS, B+)
• CHANGELOG.md updated