Adopt swift-format for linting and formatting

[yysskk]

Summary

Adds lint/format enforcement using the Swift toolchain's first-party swift format (swiftlang/swift-format, bundled since Swift 6) — no third-party lint dependency.

• .swift-format: 4-space indentation, 160-column line length, respectsExistingLineBreaks so adoption churn stays minimal; everything else is the official default rule set.
• One-time normalization of Sources, Tests, the example app, and Package.swift with swift format --in-place: bracket indentation (the codebase had misaligned ]) closers), trailing commas in multiline collections, one variable per declaration, ordered imports (@testable last), and blank-line normalization. Purely mechanical.
• Manual fixes for things the formatter does not rewrite:
  • The four .forEach { … } side-effect loops flagged by ReplaceForEachWithForLoop are now for-in loops.
  • First commit: wrapped 160+ character lines (worst was 301 chars), renamed single-letter locals (i → index, m → maskedCorners), dropped redundant = nil initializations, removed a stale 2017 TODO.
• CI job swift-format (macos-26, same setup-xcode selection as the build jobs so CI's swift-format version matches contributors' local toolchain):
  1. swift format lint --strict --recursive --parallel …
  2. swift format --in-place … + git diff --exit-code — fails if the tree is not formatter-clean and surfaces the diff in the log.

Existing job names are unchanged, so the required status checks on main are unaffected.

Verification

• swift format lint --strict passes with zero findings (swift-format 6.3.0), and the formatter is idempotent on the tree.
• All 70 package tests and 8 example tests pass locally on the iPhone 17 simulator (Xcode 26.5).

[claude]

Reviewed the full diff. This is a mechanical formatting-only change: adds a .swift-format config, applies swift format --in-place across Sources/Tests/Example, converts the four flagged .forEach side-effect loops to for-in, and adds a CI job that lints and verifies formatter-cleanliness.

Spot-checked the non-trivial rewrites for behavior preservation:

• The wrapped multi-line string literal in SwipeMenuViewController.swift's assertionFailure uses a trailing \ to suppress the line break, so the message text is unchanged.
• The padding/height local-variable hoists in TabView.swift (indicator positioning, circle indicator frame) compute the same values as before, just de-duplicated.
• Trailing commas added to multi-line arrays/NSLayoutConstraint.activate([...]) calls and import reordering are semantically inert in Swift.
• The new format CI job is independent (no needs: wiring), so it doesn't affect the existing required checks.

No bugs, logic changes, or security issues found. Approving.