feat(terminal): report text-area size via XTWINOPS (CSI 14/18 t)

[roramirez]

Summary

Implements the XTWINOPS text-area size reports:

• CSI 18 t → CSI 8 ; rows ; cols t (size in character cells).
• CSI 14 t → CSI 4 ; height ; width t (size in pixels).

fzf and tmux query these to compute layouts and preview windows. Cell size in cells was already known to the parser; the pixel report additionally needs the cell pixel size, which is carried from the main thread.

Changes

• src/terminal/parser.rs: new 't' CSI arm delegating to handle_xtwinops, which replies for 14/18 and ignores other window operations. Also collapses the DECSET/DECRST (h/l) arms of handle_dec_private_modes into ('h' | 'l', …) patterns (same behavior, fewer lines).
• src/terminal/grid.rs: add Grid.cell_px: (u32, u32), mirrored from FontMetrics on resize.
• src/drain.rs: extend the non-blocking resize signal to (cols, rows, cell_w_px, cell_h_px) via a new PendingResize type alias; apply the cell size onto the grid at both resize-application sites.
• src/pane_ops.rs, src/app_state.rs: use the PendingResize alias; populate the cell size from entry.metrics when signaling a resize.
• Tests: xtwinops_reports_text_area_size (cells, pixels, and an unsupported op); update the pending_resize tuple in pane_ops_test.rs / drain_test.rs.
• CHANGELOG.md: entry under [Unreleased] / Added.

How to test

• cargo test (see xtwinops_reports_text_area_size; full suite green).
• In a running mmterm session: printf '\033[18t' replies \033[8;<rows>;<cols>t; printf '\033[14t' replies with the pixel size.

Checks

• cargo fmt
• cargo clippy --locked -- -D warnings
• cargo test (1204 passed)
• [~] km score --trend origin/main --fail-if-worse — grade B+ (passes .kimun.toml fail_below = B); trend is -0.01 at the rounding floor, from the unavoidable indented plumbing added inside the parser event loop. Offset as far as reasonable by simplifying the DEC private-mode setter; further clawback would require gratuitous refactoring of unrelated code.
• CHANGELOG.md updated