Add terminal shell and CLI UI mode

[zcourts]

Summary

• add a new fission-shell-terminal crate that runs the normal Fission build/lower/layout pipeline and renders Core IR to terminal cells
• expose the terminal shell through the fission facade behind the terminal-shell feature
• add fission ui with screenshot/one-shot rendering support for visual verification
• render interactively only after input or resize events, avoiding continuous clear/repaint flicker

Architecture notes

• terminal support is verified from Core IR and semantics, not widget-name mappings
• unsupported graphical-only operations fail before render instead of falling back to a non-Fission TUI model
• the CLI UI is itself a Fission app rendered through fission-shell-terminal
• test-protocol integration is not forced here; the shell exposes normal InputEvent dispatch and deterministic frames so a protocol adapter can be layered later without changing Fission architecture

Verification

• cargo test -p fission-shell-terminal -p fission-cli
• cargo check -p fission --features terminal-shell
• cargo run -p fission-cli --bin fission -- ui --project-dir documentation --screenshot target/fission/ui-mode.png --exit-after-render --width 110 --height 34
• cargo run -p fission-cli --bin fission -- --help
• cargo test --workspace

Screenshot

• local verification screenshot: target/fission/ui-mode.png

Stacked on #25 (reorganize facade feature boundaries).