🔒️ Restore macOS security-scoped bookmarks

[vinceglb]

Closes #590

Summary

• Store versioned native macOS bookmark data on JVM and Kotlin/Native, with explicit legacy migration metadata.
• Use supported CoreFoundation and Security APIs on JVM; macOS-only sandbox detection now fails closed.
• Keep security-scoped access shared by normalized descendants, balanced across operations, and alive until returned streams close; release rejects new access after existing handles drain.
• Preserve the Apple data-class model and captured JVM ABI surface.
• Document macOS entitlements, refresh behavior, legacy reselection, and release lifecycle.

Validation

• ./gradlew :filekit-core:jvmTest --no-daemon --console=plain
• ./gradlew :filekit-core:macosArm64Test --no-daemon --console=plain
• ./gradlew check assemble --no-daemon --console=plain (passed, 9m 8s)
• Signed Kotlin/Native and JVM macOS relaunch acceptance was completed during the original implementation: persisted external-directory bookmarks restored after process termination and wrote files again without reopening a picker. The JVM check used normal Compose JAR native-library signing semantics.

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[chatgpt-codex-connector]

Serialize access to the shared native bookmark lease

When multiple threads operate on files derived from the same Kotlin/Native macOS bookmark, start(), stop(), and release() concurrently mutate released, activeAccesses, and scopeUrl without synchronization. For example, release() can observe zero active accesses and clear scopeUrl after start() passes its check but before it increments the counter, causing the subsequent stop() to throw and leave the native scope unbalanced; two simultaneous closes can likewise decrement the counter twice. The corresponding JVM implementation synchronizes these transitions, so the native lease needs equivalent serialization.

AGENTS.md reference: AGENTS.md:L12-L12

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