fix: make persisted cache identities portable

[lzehrung]

Summary

• Store persisted index identities relative to the active project root and migrate legacy absolute-path artifacts.
• Cover moved workspaces, resolver-environment fingerprints, bloom validation, and language-extension preservation.
• Confine persisted cache paths before reuse, canonicalize duplicate handles, and preserve external reexport specifiers.

Verification

• Focused cache, migration, native, lint, and portability regression coverage is included on the branch.
• Consolidated validation will run through GitHub Actions before review.

[lzehrung]

Superseded by #261, which now contains the portable-cache and hot-path work with passing Linux and Windows CI.