Adopt default main actor isolation and drop CocoaPods - #55

Alternative to #54.

Motivation

#54 isolates CellKit's UIKit-facing API to the main actor by annotating each declaration with @MainActor. The goal is right — CellKit is a UITableView/UICollectionView data source layer and every one of its protocols is only ever exercised on the main thread — but Swift 6.2 ships a setting built exactly for this (SE-0466), and Apple's WWDC25 guidance points UI-focused modules at it directly.

Change

defaultIsolation set to MainActor plus the Swift 6 language mode on both targets:

let swiftSettings: [SwiftSetting] = [
    .swiftLanguageMode(.v6),
    .defaultIsolation(MainActor.self),
    .enableUpcomingFeature("NonisolatedNonsendingByDefault"),
    .enableUpcomingFeature("InferIsolatedConformances")
]

Zero @MainActor annotations in the library sources. The only Sources/ changes are removals: redundant @available(iOS 13.0, tvOS 13.0, *), #if SWIFT_PACKAGE guards, and two long-standing SwiftLint violations.

Notably it also needs no rework of DifferentiableCellModelWrapper — no MainActor.assumeIsolated, no eagerly stored differenceIdentifier. Under SE-0470 the wrapper's Equatable/Differentiable conformances become main actor-isolated and DifferenceKit's nonisolated generic algorithm accepts them, because it is called from the main actor.

Other changes:

• Platforms → iOS 15 / tvOS 15, matching FTAPIKit.
• CocoaPods removed in favour of the Swift package (CellKit.podspec, Gemfile, Gemfile.lock). The podspec could not express defaultIsolation without duplicating it as SWIFT_DEFAULT_ACTOR_ISOLATION in pod_target_xcconfig, which would leave two distribution channels able to drift apart on the public concurrency contract.
• Example app moved to the Swift 6 language mode with default main actor isolation and approachable concurrency, matching how our apps are configured.
• CI rewritten to match FTAPIKit's ci.yml: no pod lib lint, actions/checkout@v6, and a build of both library schemes plus the Example app. swiftlint --strict now passes — it did not on main.

Consumer impact

Verified against a real consumer module compiled in the Swift 6 language mode:

The first row is the case #54 was written for — the 45 hand-written @MainActor conformances and the @preconcurrency import all become unnecessary.

The second row is a rough edge in conformance isolation inference: it is not inferred through inherited protocol requirements even with InferIsolatedConformances enabled. It is not introduced by this PR — main already requires an annotation there today. Both cases are documented in the README, and the Example app demonstrates the second.

Verification

• xcodebuild build succeeds for CellKit and DiffableCellKit on generic/platform=iOS with zero warnings, in the Swift 6 language mode.
• Example app builds clean (aside from pre-existing UITableViewRowAction deprecations in its own code).
• swiftlint --strict: 0 violations.

Tested with Xcode 26.6 / Swift 6.3.3.

Breaking

Source-breaking for any conformance that is not main actor-isolated, and the platform floor moves to iOS 15 / tvOS 15 with Xcode 26+ required to build. Warrants a 1.0.0 tag — the README already points at from: "1.0.0", so that tag needs cutting before this is announced.

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