Fix Swift 6 concurrency compatibility

[prikshitgupta]

What

• Adds checked Sendable conformance to SwiftLAME's immutable public value types.
• Keeps AVAudioFile local to the encoding task instead of transferring it across isolation.
• Preserves source validation during SwiftLameEncoder initialization.
• Replaces the placeholder test with Swift 5/6 compatibility, invalid-input, playable-output, and cancellation tests.

Why

Swift 6 rejects LameVbrMode.default because the enum is not Sendable, then rejects SwiftLameEncoder.encode because the task captures a struct containing AVAudioFile. Marking the encoder @unchecked Sendable would hide a real unsafe transfer, so this patch narrows stored state to checked Sendable values and constructs the audio file inside the worker task.

Behavior preserved

• Initialization still throws when the source cannot be opened.
• Encoding still runs in a task with the requested priority.
• Progress and cancellation behavior remain unchanged.
• Telephony output remains playable at 8 kHz / 64 kbps.

Verification

• swift test — 4 tests passed.
• swift test -Xswiftc -swift-version -Xswiftc 6 — 4 tests passed.
• Tests cover checked Sendable contracts, missing input, playable MP3 output, progress completion, and cancellation cleanup.
• No @unchecked Sendable, nonisolated(unsafe), or global-actor workaround is used.