Add Thistlethwaite+ solver mode

[cypherair]

Summary:

• Add a separate Thistlethwaite+ mode that keeps the four-phase reduction but optimizes across phase exits.
• Keep classic Thistlethwaite behavior unchanged.
• Add stage-aware boundary merge, optimized solver tests, release distribution gates, and opt-in long benchmark tests.
• Bump CURRENT_PROJECT_VERSION from 11001 to 11002 in a separate commit.

Verification:

• swift test --filter ThistlethwaiteSolverTests
• swift test -c release --filter ThistlethwaiteSolverTests
• swift test -c release
• xcodebuild -project cubeone.xcodeproj -scheme cubeone -destination 'platform=macOS' build