tui: add Loro web dashboard, process lifecycle, and runtime resize

Build out the tui crate (and its Python binding) into something you can
drive interactive programs with in place of a raw shell.

Dashboard (feature = "dashboard"): tui::serve mirrors every live terminal
into one loro::LoroDoc and streams CRDT updates to browsers over SSE; the
page imports them with loro-crdt and paints a grid. Loro is the view-sync
layer only; the PTYs stay authoritative in-process. Exposed to Python as
tui.serve(). The whole engine is Rust; Python is a thin binding.

Process lifecycle: the actor now owns the child, so it reaps it (capturing
the exit code through a watch channel), answers a Kill command with SIGKILL,
and reports is_alive/exit_state/wait. TuiManager::remove drops a handle, and
the Python close()/context-manager exit force-kills so an editor that
ignores Ctrl+C still goes away.

Runtime resize: resize() drives the PTY ioctl (SIGWINCH) and the vt100
emulator together; size is shared across handle clones so every clone agrees.

Empty screen is no longer an error: read_viewport returns an empty Vec and
read_blocking polls on content, so snapshot()/wait_for stay total on a blank
screen.

Author: andrewgazelka

.gitignore

@@ -28,6 +28,9 @@ __pycache__/
 *$py.class
 .venv/
 *.egg-info/
+# Compiled PyO3 extension modules dropped in-tree by `maturin develop`
+*.abi3.so
+*.pyd
 .pytest_cache/
 .mypy_cache/
 .ruff_cache/