Normalize Swift toolchain and use raw identifiers for test names

[RISCfuture]

Toolchain settings

Package.swift now points every target at a single upcomingFeatures array:

• NonisolatedNonsendingByDefault, InferIsolatedConformances (already enabled)
• ImmutableWeakCaptures, MemberImportVisibility, ExistentialAny, InternalImportsByDefault (new)

The tools version stays at 6.2 and the package keeps swiftLanguageModes: [.v5, .v6], so
existing consumers are unaffected.

Fallout, all mechanical:

• InternalImportsByDefault — eight files whose public API exposes Foundation types now say
  public import Foundation (DOF, Cycle, Obstacle, AccuracyCategory, ByteInitializable,
  DOFError, and ByteParsing, the last because its @inlinable toString() calls
  String(bytes:encoding:)). Obstacle.swift also gets public import CoreLocation for the
  coreLocation accessor. Every other file keeps Foundation internal.
• ExistentialAny — 15 any annotations added by swift package migrate, across the library
  and the SwiftDOF_E2E tool.
• ImmutableWeakCaptures — no occurrences.

Raw identifiers for test names

SE-0451 makes @Test("…") display-name strings redundant, so all 59 tests name themselves:

@Test
func `parses the currency date header into a cycle`() throws

Names were regenerated as lowercase third-person verb phrases rather than the title-case noun
phrases the display strings used — "Count property" became `counts the parsed obstacles`,
"Measurement extension - height AGL" became
`exposes the height AGL as a measurement in feet`, and so on. The three suite types keep
their CycleTests / DOFTests / ObstacleTests names; none shadows a type under test.

CI

The matrix already runs macos-26 + Swift 6.2 and ubuntu-latest + Swift 6.3, both new enough for
raw identifiers, so both legs still build and test. Only the stale policy header changed: it had
said "when Swift 6.3 ships, bump 6.0→6.1", which has happened, and it now records the Swift 6.2
floor that raw identifiers impose on any leg running swift test.

Verification

Run locally on Swift 6.3.3 / Xcode 26.6:

• swift build from a clean .build — zero errors, zero warnings
• swift test — 59 tests in 3 suites, all passing
• swift format lint --strict -r . — clean
• swiftlint --strict — 0 violations in 23 files
• swift package dump-package — clean
• actionlint .github/workflows/ci.yml — clean

For reviewers

The public import set is worth a look — it was derived from the compiler's own
"not used in public declarations" warnings, iterated to a fixed point, so it should be minimal,
but it reflects what macOS compiles. LinuxLocalization.swift sits behind #if !canImport(Darwin)
and is not compiled locally; its import Foundation is left internal because the String
extension it declares is internal, which the Linux CI leg will confirm.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EtA5RwfPWtcxu2NfAHfjAr