Implement the screen saver

[winebarrel]

Ports Neco's cat to a .saver bundle, following the layout of
winebarrel/Macstify: a Sources/
target that builds NecoSaver.saver and a Preview/ host app for development,
plus the Makefile and CI that go with them.

What is carried over from Neco

Sprites.swift, the sprite expansion, the cat's brain and the litter marks come
from Neco — copied, not shared, since the
sprite data never changes and Neko.swift is settled. Two changes make them work
under ScreenSaver.framework:

• Neko reads an injected field rect instead of NSScreen. A saver gets one
  view per display and may only draw inside its own bounds.
• There is no cursor to chase behind the shield window, so the cat always runs
  Neco's wander mode. Cursor tracking is removed rather than made switchable.

The two overlay NSPanels collapse into one ScreenSaverView that paints the
background, the mess and the cats in that order, and the 60fps Timer becomes
animateOneFrame(). Settings live in ScreenSaverDefaults, not
UserDefaults.standard, which would otherwise resolve to a different domain
depending on which host loaded the bundle.

SpriteCache now vends CGImage instead of NSImage, so the engine draws into
the CGContext it is handed with no dependency on the current
NSGraphicsContext — which is what lets the preview app render offscreen.

Options

Edited through a configure sheet built in code (no nib to locate at runtime):

Option	Default	Range
Cats	1	1–8	Each one wanders on its own.
Size	64 pt	32–192 pt	Walking speed and the size of the mess scale with it.
Speed	1.00×	0.2–3.0
Background	0%	0–100%	Grey level.
Paw prints	on
Scratch marks	on

The litter ink takes the opposite end of the greyscale from the background: the
marks are a fixed dark colour in Neco, which would leave them invisible on the
black default here.

Not done

"Show the real desktop behind the cat" is not possible in a .saver — the saver
is a CGShieldingWindowLevel() window covering the screen, and a snapshot
approach would need screen-recording TCC granted to legacyScreenSaver rather
than to us. The background is a plain grey instead.

Verified

• xcodebuild build analyze passes for both schemes
• swiftlint --strict, swiftlint analyze --strict and swiftformat --lint are clean
• The built bundle loads and its principalClass resolves, i.e. @objc(NecoSaverView)
  matches NSPrincipalClass
• Offscreen renders check the black and white backgrounds, several cats at once,
  and the Options sheet layout
• The .saver contains only Info.plist and the executable — no entitlements
  copied over from the app, no resources

Not verified: behaviour in System Settings on a real machine (make install,
then killall legacyScreenSaver), multi-display, and the live thumbnail preview.