fix: adopt Swift 6 language mode; resolve all concurrency issues (#27)

All three product targets (app, widget, share extension) now build under
SWIFT_VERSION = 6.0 with zero errors and zero warnings. The UITests
target stays on 5.0: XCTestCase's nonisolated setUp/init overrides
conflict with the target's MainActor default isolation under 6, and test
tooling is not shipping code.

The original seven diagnostics, plus the layers Swift 6 mode surfaced
once those cleared:

- SMTPChannel is an actor. It was implicitly MainActor while running its
  receive loop on a background queue, so parsedLines/lineWaiters/
  receiveBuffer were declared main-actor-protected and mutated off it —
  concurrent mutation while resuming a CheckedContinuation can
  double-resume, which traps. The actor serialises all state; Network
  callbacks hop in via Task. The start() continuation also gains an
  OSAllocatedUnfairLock resume-once guard: the state handler can fire
  .ready and later .failed, and resuming twice was a pre-existing trap of
  the same family.
- CachedLookupResult is nonisolated (a value pair built inside actor
  LookupRuntime cannot have a MainActor-bound memberwise init) with
  conditional Sendable — opting out of MainActor isolation also opted out
  of the implicit Sendable that globally-isolated types get.
- PortScanService.printableBanner is nonisolated: a pure transformation
  called from the connection's queue.
- SweepActivityController stores the activity's Sendable id instead of
  the non-Sendable Activity, re-resolving via Activity.activities inside
  each fire-and-forget task, so nothing non-Sendable crosses isolation.
- App Intents' static title/description/openAppWhenRun become lets
  (get-only protocol requirements; static var is shared mutable global
  state), and the summary helpers are @MainActor to match the model
  properties they read and the perform() implementations that call them.
- ExternalDataService's ISO8601DateFormatter is nonisolated(unsafe),
  citing Apple's documented thread-safety, rather than risking a parser
  behaviour change by switching APIs with no test coverage.
- TaskMetricsDelegate.metrics is nonisolated(unsafe): written on the
  session's delegate queue, read only after the request completes, and
  URLSession guarantees didFinishCollecting precedes task completion.
- The share extension extracts the host via async/withCheckedContinuation
  instead of sending a non-Sendable completion into loadItem's @Sendable
  handler; Task inherits the view controller's MainActor so the manual
  DispatchQueue.main hop goes too.

Validated: clean Swift 6 build of all product targets, and the full
enforced 11-test audit suite green on the floor runtime — Swift 6's
runtime isolation checks ran the app through every screen without a
trap.

0 files changed, 0 insertions, 0 deletions