fix(concurrency): send command metatypes safely

[steipete]

Summary

• require SendableMetatype for ParsableCommand on Swift 6.2 and newer instead of making every mutable command instance Sendable
• retain the original Sendable requirement for Swift 6.0 and 6.1 compilers
• add a split-file compiler fixture with deliberately non-Sendable command state

Why

Commander only transfers command type metadata across isolation domains. Command instances are created, bound, and executed on the main actor. Requiring instance-level Sendable made a split-file ParsableCommand adoption synthesize a retroactive conformance, which Swift 6.4 diagnoses and plans to reject in a future language mode.

This models the actual ownership boundary without @unchecked Sendable or consumer-side mechanical conformance moves.

Proof

• swift test -Xswiftc -warnings-as-errors — 65 tests pass with Apple Swift 6.4
• Peekaboo CLI suite with this dependency change and warnings as errors — 894 core CLI tests plus 88 runtime tests pass
• clean Peekaboo CLI warnings-as-errors build using the exact compiler-gated patch
• 37 distinct Peekaboo retroactive Sendable diagnostics before; zero after
• P0–P2 autoreview clean

Swift 6.2 is the first compiler with SendableMetatype; it was not installed on the local proof host. Commander CI continues to exercise the Swift 6.2+ branch with its configured Swift 6.3.3 toolchain.

[clawsweeper]

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ClawSweeper picked this up.

Pull request received. I will update this pull request when review starts.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix(concurrency): send command metatypes safely This is item 1/1 in the current shard. Shard 0/1.

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