Add reset() extension to TabCoordinator

Motivation

NavigationStackCoordinator has a built-in reset() (Coordinator.swift:116-119) that clears path and modalCover:

public func reset() {
    path = []
    modalCover = nil
}

TabCoordinator has no equivalent for resetting selectedTab + modalCover. Apps with tab-based roots that need to reset tab selection on logout / session-end currently write per-project boilerplate.

Proposal

Add a default implementation to TabCoordinator:

extension TabCoordinator {
    /// Resets the tab coordinator: selects the given default tab and dismisses any modal cover.
    /// Child flow coordinators should be reset separately via their own `reset()`.
    public func reset(to defaultTab: Tab) {
        selectedTab = defaultTab
        modalCover = nil
    }
}

The defaultTab parameter is required because Tab is a project-specific associated type — the framework can't hardcode a sensible default.

Usage example

smsticket-ios MainTabCoordinator.resetNavigation() currently:

func resetNavigation() {
    selectedTab = .home
    modalCover = nil
    homeCoordinator.reset()       // existing NavigationStackCoordinator.reset()
    searchCoordinator.reset()
    favoritesCoordinator.reset()
    ticketsCoordinator.reset()
}

With the proposed extension, the first two lines become reset(to: .home) — consistent with the NavigationStackCoordinator pattern.

Open questions

• Should reset() cascade to child flow coordinators if a children-tracking API were introduced? Probably out of scope — each parent knows its own children and reset order can matter per-app.
• Per-tab modal covers — not currently supported by TabCoordinator anyway, so N/A.

Related

Surfaced during review on futuredapp/smsticket-ios#20 (see MainTabCoordinator.resetNavigation).