Make MultiProducerSingleConsumerAsyncChannel internals compatible with library evolution

[adityasingh2400]

MultiProducerSingleConsumerAsyncChannel, introduced in 1.1.0, fails to compile when the consuming module is built with library evolution enabled (BUILD_LIBRARY_FOR_DISTRIBUTION=YES, i.e. -enable-library-evolution). This blocks every team that distributes a binary SDK as an XCFramework depending on swift-async-algorithms; the 1.0.x line is currently the only usable version for them.

Two classes of error come from Sources/AsyncAlgorithms/MultiProducerSingleConsumerChannel/MultiProducerSingleConsumerAsyncChannel+Internal.swift:

• 'consume' can only be used to partially consume storage — the @inlinable mutating func methods on the non-@frozen _StateMachine struct (and the _InternalBackpressureStrategy enum) do switch consume self._state / switch consume self. Under library evolution a non-frozen type's stored properties are reached through resilient accessors, which do not vend the direct ownership consume requires.
• initializer for class '_Storage' is '@inlinable' and must delegate to another initializer, plus the related 'let' property ... may not be initialized directly errors on the @inlinable initializers of the ~Copyable payload structs. Under library evolution an @inlinable initializer of a resilient type may only assign through a delegating self.init/self = ..., not set stored properties directly.

These reproduce on 1.1.0–1.1.3; main is still affected.

Changes:

• Mark the internal (@usableFromInline) types whose stored storage is consumed or directly initialized in @inlinable members as @frozen, so their layout is fixed and direct storage access / consume is permitted under library evolution: _StateMachine, _State and its Channeling/SourceFinished/Finished payloads, _InternalBackpressureStrategy and its _Watermark, the ResumeElement/ResumeElementAndProducers/ResumeFailure wrappers, and SendableConsumeOnceBox. These are all already @usableFromInline internal types, so freezing them is not a public-API or source-stability change.
• Mark _TinyArray and its Storage enum @frozen for the same reason (they back the frozen payloads).
• Change _Storage.init(backpressureStrategy:) from @inlinable to @usableFromInline, since an @inlinable designated class initializer must delegate under library evolution and this one sets a stored property directly.

No public API changes and no behavioral changes — only resilience-attribute additions on internal types and one inlinability downgrade on an internal initializer.

Testing:

• swift build --target AsyncAlgorithms -Xswiftc -enable-library-evolution now succeeds (it fails on main with the errors above).
• swift build (normal, non-evolution) succeeds.
• swift test --filter MultiProducerSingleConsumer passes — 37 tests, 0 failures.

Fixes #405.