Open same-host non-HTTP(S) URLs (webcal:, mailto:, ...) via the system

[pardeyke]

Same-host URLs with a non-HTTP(S) scheme are currently routed as in-app Turbo visits, which always fail. This PR routes them through SystemNavigationRouteDecisionHandler instead, so the system opens them — matching how external-scheme links on other hosts already behave.

The problem

A link like webcal://my.app.com/calendars/subscribe.ics (calendar subscription served by the app's own backend) matches AppNavigationRouteDecisionHandler, because it compares hosts without looking at the scheme. The resulting Turbo visit can't work — Turbo.js can't even record it in history:

Error evaluating JavaScript function `window.turboNative.visitLocationWithOptionsAndRestorationIdentifier':
SecurityError: Blocked attempt to use history.pushState() to change session history URL from
http://my.app.com/calendars to webcal://my.app.com/calendars/subscribe.ics.
Protocols, domains, ports, usernames, and passwords must match.

The visit then fails with status code 0 and falls into the JS fetch recovery path, which also fails (unsupported URL), ending in an error screen.

SystemNavigationRouteDecisionHandler doesn't catch this case either: it only matches when hosts differ. Its existing tests (test_sms_urls_match, test_non_http_urls_match) show the intent is for external schemes to be opened by the system — those URLs just happen to have a nil host, so they pass the host comparison by accident. As soon as the URL carries the app's own host, the intent breaks down.

The fix

• AppNavigationRouteDecisionHandler only matches http/https URLs — it can only ever route those in-app.
• SystemNavigationRouteDecisionHandler matches any non-HTTP(S) URL regardless of host, so UIApplication.shared.open hands it to the owning app (Calendar for webcal:, Mail for mailto:, ...).

Behavior for HTTP(S) URLs is unchanged, so this shouldn't affect SafariViewControllerRouteDecisionHandler or the universal-link handling from #191 / #195. Tests added for the same-host webcal: case on both handlers; the full suite passes.

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