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/pyproject.toml

@@ -14,6 +14,9 @@ dependencies = [
     "textual>=1.0.0",
 ]
 
+[project.optional-dependencies]
+camera = ["Pillow>=10.0", "astropy>=6.0"]
+
 [project.scripts]
 birdcage-tui = "birdcage_tui.app:main"