Add Camera Capture overlay (F6) with multi-trigger capture pipeline

Camera backend abstraction wrapping fswebcam/ffmpeg/libcamera-still via
subprocess, with DemoCamera fallback generating valid JPEG from raw bytes.

Capture pipeline writes JPEG + JSON sidecar always, optional FITS (astropy)
and EXIF (Pillow) when available. Thread-safe orchestrator with session
tracking, sequence numbering, and date-based output directories.

Trigger system: manual capture, configurable interval timer, and pass event
detection (AOS/TCA/LOS) with 0.5-degree TCA hysteresis. PassEventDetector
runs in the Craft tracking loop, fires callbacks to the camera overlay.

F6 overlay follows the F5 ConsoleOverlay pattern — ModalScreen with
install_screen persistence. Status panel, scrollable capture log, interval
controls, and AOS/TCA/LOS toggle buttons.

Tagline: "a generic AZ/EL positioner that doesn't care about wavelength"
added to TUI header subtitle.

51 new tests (77 total passing).

tui/tests/test_camera_backend.py

@@ -0,0 +1,113 @@
+"""Test camera backend abstraction — DemoCamera, SubprocessCamera, detection.
+
+DemoCamera is always available and produces valid output files.
+SubprocessCamera reports unavailable when the tool doesn't exist.
+"""
+
+from pathlib import Path
+
+import pytest
+
+from birdcage_tui.camera import (
+    CameraConfig,
+    DemoCamera,
+    SubprocessCamera,
+    auto_select_backend,
+    detect_cameras,
+)
+
+
+def test_demo_camera_captures_file(tmp_path: Path):
+    """DemoCamera should produce a file at the requested path."""
+    camera = DemoCamera()
+    output = tmp_path / "test.jpg"
+
+    result = camera.capture(output)
+
+    assert result.success
+    assert output.exists()
+    assert output.stat().st_size > 0
+    assert result.timestamp  # ISO-8601 UTC string
+    assert result.duration_ms >= 0
+
+
+def test_demo_camera_always_available():
+    """DemoCamera.is_available() should always return True."""
+    camera = DemoCamera()
+    assert camera.is_available()
+
+
+def test_demo_camera_name():
+    """DemoCamera.name should be 'demo'."""
+    camera = DemoCamera()
+    assert camera.name == "demo"
+
+
+def test_demo_camera_valid_jpeg(tmp_path: Path):
+    """DemoCamera should produce a file starting with JPEG SOI marker."""
+    camera = DemoCamera()
+    output = tmp_path / "test.jpg"
+
+    result = camera.capture(output)
+    assert result.success
+
+    data = output.read_bytes()
+    # JPEG files start with SOI marker: FF D8
+    assert data[:2] == b"\xff\xd8"
+
+
+def test_demo_camera_custom_resolution():
+    """DemoCamera should accept custom resolution."""
+    camera = DemoCamera(resolution=(640, 480))
+    assert camera._resolution == (640, 480)
+
+
+def test_subprocess_camera_not_available():
+    """SubprocessCamera with nonexistent tool should be unavailable."""
+    config = CameraConfig(backend_cmd="nonexistent_camera_tool_xyz")
+    camera = SubprocessCamera(config)
+    assert not camera.is_available()
+
+
+def test_subprocess_camera_capture_missing_tool(tmp_path: Path):
+    """Capture with missing tool should return failure."""
+    config = CameraConfig(backend_cmd="nonexistent_camera_tool_xyz")
+    camera = SubprocessCamera(config)
+    output = tmp_path / "test.jpg"
+
+    result = camera.capture(output)
+    assert not result.success
+    assert result.error  # Has a meaningful error message
+
+
+def test_subprocess_camera_name():
+    """SubprocessCamera.name should include tool and device."""
+    config = CameraConfig(backend_cmd="fswebcam", device="/dev/video0")
+    camera = SubprocessCamera(config)
+    assert "fswebcam" in camera.name
+    assert "/dev/video0" in camera.name
+
+
+def test_detect_cameras_returns_list():
+    """detect_cameras() should return a list (may be empty in CI)."""
+    devices = detect_cameras()
+    assert isinstance(devices, list)
+
+
+def test_auto_select_returns_available_backend():
+    """auto_select should return an available backend (demo or real)."""
+    config = CameraConfig(backend_cmd="nonexistent_tool_xyz")
+    backend = auto_select_backend(config)
+    # Should always return something available
+    assert backend.is_available()
+    # If no real backends exist, should be demo; otherwise a real one
+    assert backend.name  # Has a name
+
+
+@pytest.mark.parametrize("backend_cmd", ["fswebcam", "ffmpeg", "libcamera-still"])
+def test_subprocess_unknown_backend_fails(tmp_path: Path, backend_cmd: str):
+    """Unknown backend command pattern should be in the lookup table."""
+    config = CameraConfig(backend_cmd=backend_cmd)
+    SubprocessCamera(config)
+    # The command template should exist even if the tool isn't installed
+    assert backend_cmd in SubprocessCamera._COMMANDS