Make PrivateObjectActor package-visible

[julianschiavo]

Summary

• make PrivateObjectActor package-visible
• make PrivateObjectActor.shared package-visible so package-level consumers in XCDocs and XCDocsSupport can link against XCDocsBridge

Validation

• swift build --package-path /tmp/xcdocs-link-fix --cache-path /tmp/xcdocs-cache --config-path /tmp/xcdocs-config --security-path /tmp/xcdocs-security --scratch-path /tmp/xcdocs-scratch --manifest-cache local --configuration release --product XCDocs
• swift build --package-path /tmp/xcdocs-link-fix --cache-path /tmp/xcdocs-cache --config-path /tmp/xcdocs-config --security-path /tmp/xcdocs-security --scratch-path /tmp/xcdocs-scratch-x86 --manifest-cache local --configuration release --triple x86_64-apple-macosx15.0 --product XCDocs

The missing archive symbol was XCDocsBridge.PrivateObjectActor.shared.unsafeMutableAddressor, referenced from both XCDocs and XCDocsSupport.

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Note

Low Risk
Low risk: only changes Swift access control for PrivateObjectActor and PrivateObjectActor.shared, with no behavioral logic changes.

Overview
Exposes the @globalActor PrivateObjectActor and its shared singleton as package to fix cross-target linkage for package-level consumers of XCDocsBridge (e.g., references to PrivateObjectActor.shared).

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