Swap alacritty_terminal for rio-vt

[raphamorim]

This swaps the VT emulation backend from alacritty_terminal to rio-vt, the terminal core we extracted from Rio. The PTY layer (portable-pty) is untouched, so this is purely an engine swap behind Emu.

What changed:

• emu.rs now wraps rio_vt::crosswords::Crosswords. Cell colors/attributes are resolved from rio-vt's packed cell + per-grid style table, mapped to the same EmuCell/Color surface the rest of the code consumes.
• CommandTracker in integration.rs keeps its exact logic; it just implements rio-vt's Perform trait instead of vte's (same state machine lineage, same callback shape, so the diff there is one import line).
• Terminal replies (DA/DSR/CPR) come from rio-vt as events and flow through the same pending-writes path as before.

Why: rio-vt is actively maintained, it's the engine we run in production in Rio, and it brings correct unicode width/grapheme handling plus shell-integration parsing (OSC 7/133) natively should shell-use want to lean on more of it later. It also drops the winit-adjacent transitive deps that alacritty_terminal pulls in. Throughput is on par with or ahead of alacritty_terminal on parsing-heavy workloads; benchmarks against alacritty_terminal and vt100 live at https://github.com/raphamorim/rio-vt-benchmark.

Verified: all 48 tests pass, clippy clean. I also drove the real daemon on macOS: session open, submit/wait command, exit-code + cwd tracking (OSC 133/7), bold/color/underline cell attributes via cells, and wide characters (CJK with correct spacer cells). Screenshot/SVG rendering works.

One pre-existing quirk I noticed while testing (not from this change; reproduces on main): with zsh the captured command can double the first character ("pprintf ..."), which seems to come from prompt repaint echo. Happy to dig at that separately.