Release Spiderweb v0.5.6

[DeanoC]

Summary

• bump Spiderweb to v0.5.6
• document the hosted-registry e2e, protocol-path, and registry metadata integrity fixes in the changelog
• update the macOS marketing version and runtime version strings for the new patch release

Testing

• ./scripts/check-version-sync.sh
• zig build test

[chatgpt-codex-connector]

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Reviewed commit: 6415b48cb7

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[DeanoC]

Fixed the release review comment by updating current_spiderweb_version in src/acheron/control_plane.zig from 0.5.5 to 0.5.6, so registry install/update compatibility checks match the release line. Revalidated with ./scripts/check-version-sync.sh and zig build test.