chore(rust): delete fleet-tui-poc throwaway crate (#165)

The fleet-tui-poc crate's README declared it throwaway: "Once
fleet-ui lands in rust/fleet-ui/, delete this directory."
fleet-ui has since shipped the canonical overlay widgets:

- `fleet-ui::overlay`            — base overlay framework
- `fleet-ui::spotlight_overlay`  — search-first palette (was POC key `2`)
- `fleet-ui::session_switcher_overlay` — card-stack switcher (was POC key `4`)

The POC carried ~3470 lines of duplicate SpotlightItem / context-menu
/ action-sheet / session-switcher code drifting from those canonical
implementations. Deleting it removes the duplication, slims the
workspace Cargo.lock (fleet-tui-poc entry gone), and avoids a
ratatui-version split risk should the POC and fleet-ui ever diverge.

Workspace `members = ["fleet-input", "fleet-layout", "fleet-*"]` glob
picks crates up by directory presence, so removing the directory is
enough — no manifest edit needed.

Three live shell scripts referenced the binary; updated:

- bin/pane-context-menu-chooser.sh — dropped Rust branch; bash menu
  is now the sole renderer (always was the fallback).
- bin/section-jump-chooser.sh — converted to a tmux-display stub.
  The overlay had no bash fallback; a fleet-ui-backed replacement
  is a follow-up.
- full-bringup.sh — dropped the dead design-preview window block
  that was gated on the binary's presence.

Verification:
- cargo check --workspace: clean
- cargo test --workspace --exclude fleet-ui: all green (217 tests)
- cargo test -p fleet-ui --lib: 39/39 pass
- Pre-existing broken `tab_strip_snapshot.rs` integration test
  (imports nonexistent `fleet_ui::tab_strip`) was failing on HEAD
  before this PR; unrelated to this change.

Co-authored-by: NagyVikt <nagy.viktordp@gmail.com>

Author: NagyVikt