Adopt default main actor isolation and drop CocoaPods

[jmarek41]

Supersedes #54 (left open for its author; see the comment there). Credit to @radimvaculik for the analysis that motivated this.

Motivation

CellKit is a UITableView/UICollectionView data-source layer — every one of its protocols is only ever exercised on the main thread. Under Swift 6 that mismatch surfaces as conformance of 'FooCellModel' to protocol 'CellConvertible' crosses into main actor-isolated code on every consumer conformance (61 warnings in one app, 45 hand-written @MainActor conformances to silence them).

Change

defaultIsolation set to MainActor with the Swift 6 language mode on both targets (SE-0466; the setting Apple's WWDC25 guidance recommends for UI-focused modules):

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

Zero @MainActor annotations in Sources/. The only Sources/ edits are removals: redundant @available(iOS 13, *), #if SWIFT_PACKAGE guards, two long-standing SwiftLint violations.

• Platforms → iOS 15 / tvOS 15, matching FTAPIKit. Requires swift-tools-version:6.2 → Xcode 26+.
• CocoaPods removed (CellKit.podspec, Gemfile, Gemfile.lock). Swift Package only from 1.0.0. Removing Gemfile.lock also clears the 11 open rubygems security alerts; Bump cocoapods-downloader from 1.3.0 to 1.6.3 #51–Bump activesupport from 4.2.11.3 to 7.2.3.1 #53 were closed as superseded.
• DifferenceKit floor → 1.3.0 (the only release with the current API, already what Package.resolved pinned).
• Example app: Swift 6 language mode, SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor, Approachable Concurrency — how our apps are configured. Adopts the UIScene lifecycle (iOS 27 terminates apps that don't). Fixes cellHeight declared as CGFloat — it never satisfied the Double requirement, so every row silently fell back to 44pt.
• Example UI tests actually run now. The project had no shared scheme, so xcodebuild test had never built ExampleUITests. Added Example.xcscheme, and replaced the XCTAssert(true) placeholder with three tests that drive the main actor-isolated DifferentiableCellModelDataSource on a real table: sections render; tapping a cell updates it through a diff; inserting rows and resetting diffs correctly.
• CI rewritten to match FTAPIKit's ci.yml (actions/checkout@v7, brew install swiftlint, swiftlint --strict), with xcodebuild in place of swift build/swift test because CellKit is UIKit-only: both library builds, then the Example UI tests on a simulator chosen dynamically (newest iOS runtime, first iPhone) so runner image changes don't break it. Job id stays test (the required check on main).

Comparison with #54

#54 annotates each declaration by hand and reworks DifferentiableCellModelWrapper with MainActor.assumeIsolated. Building that branch in the Swift 6 language mode still fails in DiffableCellKit:

1. ==: sending 'lhs' risks causing data races — non-Sendable wrappers captured into the @MainActor closure.
2. arraySection: cells.map(DifferentiableCellModelWrapper.init) passes a @MainActor init as a bare function value — loses global actor 'MainActor'.
3. Nested Container.reload doesn't inherit the class's @MainActor and calls UIKit from a nonisolated context.

It also leaves CellKit.podspec at iOS 9, which cannot compile MainActor. With defaultIsolation none of the wrapper rework is needed: SE-0470 isolated conformances let DifferenceKit's nonisolated generic algorithm accept the main actor-isolated wrapper because it is always called from the main actor.

Consumer impact

Verified with a real consumer module against this branch:

Consumer module (Swift 6)	main today	This branch
Default nonisolated isolation (#54's case)	1 annotation per model type	none
SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor	1 per model type	@MainActor per CellKit protocol conformed to (Xcode 26); none on Xcode 27 / Swift 6.4, which infers them

The second row is an SE-0470 inference gap for conformances to protocols that are themselves main actor-isolated; it pre-dates this PR and is documented in the README. The Example app shows that configuration.

Verification

• swiftlint --strict: 0 violations.
• xcodebuild build for CellKit and DiffableCellKit (generic/platform=iOS): 0 warnings, Swift 6 language mode. Verified on Xcode 26.6 / Swift 6.3.3 and Xcode 27.0 beta / Swift 6.4.
• xcodebuild test Example: 3/3 UI tests pass on iPhone 17 Pro Max (iOS 27.0) and iPhone 16 (iOS 18.6), twice each with retries disabled. The app was also launched on the iPhone 16 simulator and visually checked: nav bar actions, 170pt "Welcome!" cell, "Cell Phones" header, five iPhone and five Android rows at 60pt.
• Consumer matrix above re-run with swiftc against the built module.

Breaking

iOS 15+ / tvOS 15+, Xcode 26+ to build, CocoaPods dropped. Any conformance that is not main actor-isolated stops compiling. Ships as 1.0.0 — the README already points at from: "1.0.0"; the tag is cut right after merge.

[jmarek41]

Closing for now — keeping the work on the branch while we settle the approach.