Add Textual TUI for SkyWalker-1 RF tool

Separate entry point (skywalker-tui) that reuses skywalker_lib.py
unchanged. Five RF modes: spectrum, scan, monitor, lband, track —
each with threaded USB bridge workers for non-blocking I/O.

Includes --demo mode with synthetic signal generation (Gaussian
peaks, noise floor, AGC simulation) for development without hardware.

Custom widgets: spectrum bar chart, rolling waterfall, signal gauge,
sparkline history, transponder table, device status bar.

tui/src/skywalker_tui/app.py

@@ -0,0 +1,165 @@
+"""SkyWalker-1 TUI — main application.
+
+Provides mode switching between 5 RF operating modes via a sidebar and F-key
+shortcuts. Each mode is a Screen subclass that manages its own workers.
+
+Note: We use "rf_mode" terminology for our 5 operating modes to avoid colliding
+with Textual's built-in App.mode / _current_mode / _screen_stacks system.
+"""
+
+import argparse
+import sys
+import os
+from pathlib import Path
+
+# Add tools directory to path for skywalker_lib import
+_tools_dir = str(Path(__file__).resolve().parent.parent.parent.parent / "tools")
+if _tools_dir not in sys.path:
+    sys.path.insert(0, _tools_dir)
+
+from textual.app import App, ComposeResult
+from textual.binding import Binding
+from textual.containers import Horizontal, Vertical
+from textual.widgets import Header, Footer, Button, Label, Static, ContentSwitcher
+
+from skywalker_tui.bridge import USBBridge
+from skywalker_tui.demo import DemoDevice
+from skywalker_tui.widgets.status_bar import DeviceStatusBar
+
+from skywalker_tui.screens.spectrum import SpectrumScreen
+from skywalker_tui.screens.scan import ScanScreen
+from skywalker_tui.screens.monitor import MonitorScreen
+from skywalker_tui.screens.lband import LBandScreen
+from skywalker_tui.screens.track import TrackScreen
+
+
+MODES = {
+    "spectrum": ("F1 Spectrum", SpectrumScreen),
+    "scan": ("F2 Scan", ScanScreen),
+    "monitor": ("F3 Monitor", MonitorScreen),
+    "lband": ("F4 L-Band", LBandScreen),
+    "track": ("F5 Track", TrackScreen),
+}
+
+
+class SkyWalkerApp(App):
+    """Textual TUI for Genpix SkyWalker-1 DVB-S receiver."""
+
+    TITLE = "SkyWalker-1"
+    SUB_TITLE = "DVB-S RF Tool"
+    CSS_PATH = "theme.tcss"
+
+    BINDINGS = [
+        Binding("f1", "rf_mode('spectrum')", "Spectrum", show=True),
+        Binding("f2", "rf_mode('scan')", "Scan", show=True),
+        Binding("f3", "rf_mode('monitor')", "Monitor", show=True),
+        Binding("f4", "rf_mode('lband')", "L-Band", show=True),
+        Binding("f5", "rf_mode('track')", "Track", show=True),
+        Binding("q", "quit", "Quit", show=True),
+        Binding("d", "toggle_dark", "Theme", show=True),
+    ]
+
+    def __init__(self, bridge: USBBridge, initial_mode: str = "spectrum"):
+        super().__init__()
+        self._bridge = bridge
+        self._initial_rf_mode = initial_mode
+        self._active_rf_mode = initial_mode
+        self._rf_screens: dict[str, object] = {}
+
+    def compose(self) -> ComposeResult:
+        yield Header()
+        with Horizontal():
+            with Vertical(id="sidebar"):
+                yield Label("[bold #00d4aa]SkyWalker-1[/]", classes="sidebar-heading")
+                yield Label("[#506878]DVB-S RF Tool[/]", classes="sidebar-heading")
+                yield Static("")
+                for mode_key, (label, _cls) in MODES.items():
+                    yield Button(label, id=f"btn-{mode_key}", classes="mode-button")
+                yield Static("")
+                yield DeviceStatusBar(self._bridge)
+            yield ContentSwitcher(id="content-area")
+        yield Footer()
+
+    def on_mount(self) -> None:
+        # Initialize status bar
+        status = self.query_one(DeviceStatusBar)
+        status.update_status(self._bridge)
+
+        # Install all mode screens into the content switcher
+        switcher = self.query_one("#content-area", ContentSwitcher)
+        for mode_key, (_label, cls) in MODES.items():
+            screen = cls(self._bridge, id=f"screen-{mode_key}")
+            self._rf_screens[mode_key] = screen
+            switcher.mount(screen)
+
+        # Activate initial mode
+        self.action_rf_mode(self._initial_rf_mode)
+
+    def action_rf_mode(self, mode: str) -> None:
+        """Switch to a different RF operating mode."""
+        if mode not in MODES:
+            return
+
+        self._active_rf_mode = mode
+        switcher = self.query_one("#content-area", ContentSwitcher)
+        switcher.current = f"screen-{mode}"
+
+        # Update sidebar button highlights
+        for mode_key in MODES:
+            btn = self.query_one(f"#btn-{mode_key}", Button)
+            btn.remove_class("-active")
+        self.query_one(f"#btn-{mode}", Button).add_class("-active")
+
+        self.sub_title = f"DVB-S RF Tool — {MODES[mode][0]}"
+
+    def on_button_pressed(self, event: Button.Pressed) -> None:
+        """Handle sidebar mode button clicks."""
+        btn_id = event.button.id or ""
+        if btn_id.startswith("btn-"):
+            mode = btn_id[4:]
+            if mode in MODES:
+                self.action_rf_mode(mode)
+
+    def action_toggle_dark(self) -> None:
+        self.dark = not self.dark
+
+
+def main():
+    parser = argparse.ArgumentParser(
+        prog="skywalker-tui",
+        description="Textual TUI for Genpix SkyWalker-1 DVB-S receiver",
+    )
+    parser.add_argument(
+        "--demo", action="store_true",
+        help="Use synthetic signal data (no hardware required)",
+    )
+    parser.add_argument(
+        "mode", nargs="?", default="spectrum",
+        choices=list(MODES.keys()),
+        help="Initial mode (default: spectrum)",
+    )
+    parser.add_argument(
+        "-v", "--verbose", action="store_true",
+        help="Verbose USB logging (hardware mode only)",
+    )
+    args = parser.parse_args()
+
+    if args.demo:
+        device = DemoDevice()
+        bridge = USBBridge(device)
+    else:
+        try:
+            from skywalker_lib import SkyWalker1
+            device = SkyWalker1(verbose=args.verbose)
+            device.open()
+            bridge = USBBridge(device)
+        except Exception as e:
+            print(f"Cannot open SkyWalker-1: {e}", file=sys.stderr)
+            print("Use --demo for synthetic signal data.", file=sys.stderr)
+            sys.exit(1)
+
+    app = SkyWalkerApp(bridge=bridge, initial_mode=args.mode)
+    try:
+        app.run()
+    finally:
+        bridge.close()