Makima iOS — Screenshots scheme + demo modemakima-ios-m2-auth

Adds an opt-in `Makima-Screenshots` scheme + `Screenshots` build config that
short-circuits auth with an in-process URLProtocol and seeds the app with
canned demo data. Lets us run the app past onboarding in UI tests or on a
fresh simulator without touching a real Makima server.

What lands
- ScreenshotMode: compile-time flag via -DSCREENSHOT_MODE (set in the new
  Screenshots configuration in project.yml).
- AppState: when the flag is on, calls auth.seedScreenshotData() in init.
- AuthStore: adds seedSetClient/seedSetState helpers (internal, only useful
  for screenshot + future preview builds) and a seedScreenshotData that
  wires up a fake APIClient backed by DemoSession.
- DemoSession: URLSession wired to DemoURLProtocol, which answers
  /api/v1/mesh/daemons, /contracts, /mesh/tasks, /directives,
  /listen/sessions, /mesh/tasks/{id}, /mesh/tasks/{id}/output with
  deterministic demo JSON. Host-scoped to makima.jp so production URL
  remains an operational realm.
- Tests/MakimaUITests/ScreenshotTests.swift: five XCTest cases that drive
  the app through Home, Contracts, Task detail, Directives, and Settings.
  Each attaches an XCTAttachment screenshot with a stable name.
- project.yml: adds MakimaUITests target + Screenshots configuration +
  Makima-Screenshots scheme.

Usage (on a Mac)
  make xcgen
  xcodebuild \\
    -project Makima.xcodeproj \\
    -scheme Makima-Screenshots \\
    -configuration Screenshots \\
    -destination 'platform=iOS Simulator,name=iPhone 16 Pro' \\
    test

Screenshots appear as attachments inside the resulting .xcresult bundle;
open the bundle in Xcode > Report Navigator to browse them, or use
`xcrun xcresulttool get test-results attachments` to export to PNG.

Scope
- Normal Debug/Release builds are unaffected — SCREENSHOT_MODE is only
  defined under the Screenshots configuration.
- DemoURLProtocol only matches host=makima.jp, so any real request made
  during screenshot tests (to non-makima.jp hosts) still fails normally.
- No production code path gates on ScreenshotMode beyond the init-time
  seed call; ensureWebSocket is stubbed to a fake online status so the
  masthead pill shows LIVE during screenshots.

Not included in this commit: the matching GitHub Actions workflow
(.github/workflows/ios-ci.yml) — requires a workflow-scoped token to push
and will land as a follow-up. The Swift helper that extracts screenshots
from xcresult is also queued for that follow-up.

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