Modern Swift concurrency (breaking)

[RISCfuture]

Summary

Adopts the Approachable Concurrency upcoming-feature flags
(NonisolatedNonsendingByDefault, InferIsolatedConformances) across the
library, test, and E2E targets, and migrates the codebase off legacy
concurrency constructs onto plain structured concurrency.

• Removed the downloader's GCD DispatchQueue.main.async hop and the per-chunk
  Task { @MainActor in … } hops in the archive and directory distributions,
  replacing them with direct, thread-safe Progress mutation. This also removes
  a hazard where progress could be reported out of order or after a stream had
  finished.
• Replaced the two nonisolated(unsafe) statics in the test URL-protocol mock
  with an OSAllocatedUnfairLock (chosen over Synchronization.Mutex, which
  requires macOS 15 vs the package's macOS 13 floor).
• Modernized the manual smoke-test harness from RunLoop.main.run() to
  top-level await.

Kept intentionally: UnitSlope's @unchecked Sendable (Foundation
Dimension), the @preconcurrency import ZIPFoundation lines (load-bearing for
the non-Sendable Archive), and the @preconcurrency import RegexBuilder
lines (Reference<…> is still non-Sendable; the build proved them
load-bearing).

Breaking changes

JSONZipEncoder and JSONZipDecoder changed from open
JSONEncoder/JSONDecoder subclasses marked @unchecked Sendable into
Sendable structs, eliminating an unsound @unchecked conformance on mutable
classes. The encoder gains init(outputFormatting: JSONEncoder.OutputFormatting = [])
and a public var outputFormatting; encode(_:) and decode(_:from:) are now
value-type methods.

This is source-breaking for anyone who:

• subclassed JSONZipEncoder/JSONZipDecoder,
• used them polymorphically as JSONEncoder/JSONDecoder, or
• set JSONEncoder/JSONDecoder properties other than outputFormatting.

The no-argument JSONZipEncoder() / JSONZipDecoder() initializers still
compile.

Verification

• swift build --build-tests succeeds with zero new warnings.
• Full unit-test suite passes (110/110).
• The SwiftNASR_E2E executable target was run end-to-end against live FAA
  data and passed.

Version

This change set includes a source-breaking change — JSONZipEncoder and
JSONZipDecoder are now Sendable value types (struct) instead of
JSONEncoder/JSONDecoder subclasses. The previously-unreleased [3.1.0]
CHANGELOG section (which had only additive changes) has therefore been promoted
to [4.0.0], folding in those pending additions plus this migration. The
GitHub release will be cut after merge.