Add SwiftLint

[yysskk]

Summary

Adds linting to the harness so style and hygiene regressions are caught in CI rather than in review.

• .swiftlint.yml covers Sources, Tests, and the example app. The config deviates from SwiftLint's defaults only where this codebase's structure is deliberate, with a comment for each: option types nest by component (nesting: type_level: 3), tests keep windows alive with withExtendedLifetime((a, b, c)) tuples (large_tuple), two-letter names like vc and the conventional r/g/b/a color components are allowed (identifier_name), and the line/file/type-length thresholds match the current shape of the code (160 / 600 / 350).
• CI job SwiftLint on ubuntu-latest using the pinned official docker image (ghcr.io/realm/swiftlint:0.64.1) with --strict (warnings fail) and the github-actions-logging reporter for inline annotations. Existing job names are untouched, so the required checks on main are unaffected.
• Violation fixes the default rules caught, all behavior-preserving:
  • Wrapped the remaining 160+ character lines (the worst was 301 characters) in TabView, SwipeMenuView, and SwipeMenuViewController.
  • Renamed single-letter locals (i → index, m → maskedCorners).
  • Dropped redundant = nil optional initializations in the options.
  • Removed the stale 2017 // TODO: case infinity comment (never implemented, no issue tracks it).
  • Trailing whitespace/newline fixes.

Tests

• swiftlint lint --strict passes locally with zero violations (SwiftLint 0.64.1).
• All 70 package tests and 8 example tests pass locally on the iPhone 17 simulator (Xcode 26.5).

[yysskk]

Closing in favor of the first-party toolchain formatter: the lint/format setup will use apple/swift-format (swift format from the Swift 6 toolchain) instead of SwiftLint. The unrelated code fixes from this branch (line wrapping, naming, stale TODO) will be carried over to the replacement PR.