refactor(whatsrust): remove final facade-owned definitions

[Andiveli]

Summary

• Move the last constant and callback translator implementations out of whatsrust/src/lib.rs.
• Keep the existing public API and ABI while making the facade declarations and explicit reexports only.
• Closes refactor(whatsrust): remove final facade-owned definitions #336.

Changes

• Moved CallbackTranslator<CJID> for JID to models.rs.
• Moved CallbackTranslator<i64> for i64 to registrations.rs.
• Moved VIEW_ONCE_UNAVAILABLE_DESCRIPTION to models.rs and reexported it.
• Relocated required CString imports to their owning modules and removed the lib.rs residue.

Chain Context

• Immediate parent: PR refactor(whatsrust): remove residual facade test copies #335, exact base refactor/whatsrust-facade-final at 7a4d8b1.
• Diagram: PR refactor(whatsrust): remove residual facade test copies #335 -> 📍 this PR.
• Start: PR refactor(whatsrust): remove residual facade test copies #335 leaves residual callback translators, a public model constant, and a shared CString import in the facade.
• End: whatsrust/src/lib.rs is a 54-line module-and-reexport facade with no constants, impls, unsafe blocks, or logic.
• Follow-up: none; this is the final facade cleanup slice.
• Out of scope: behavior, ABI, and unrelated refactors.

Testing

• cargo fmt --all -- --check
• CARGO_TARGET_DIR=/home/samael/Escritorio/public/wptui-public/target cargo check --workspace --all-targets
• CARGO_TARGET_DIR=/home/samael/Escritorio/public/wptui-public/target cargo test -p whatsrust --lib -- --test-threads=1 (24 passed)
• CARGO_TARGET_DIR=/home/samael/Escritorio/public/wptui-public/target cargo test --test whatsrust_api_contract -- --test-threads=1 (1 passed)
• go test ./... and go vet ./... in whatsrust/lib (passed)
• git diff --check
• Full cargo test --workspace -- --test-threads=1: attempted, but the shared filesystem had no free space (No space left on device).
• Manual/runtime testing: N/A; this is a source ownership refactor with no behavior change.

Review Budget and Rollback

• Authored diff: 46 lines (25 additions + 21 deletions), under the 400-line budget.
• Commit: 5cbbd3d.
• Rollback: revert 5cbbd3d; this removes only the final facade ownership cleanup and restores PR refactor(whatsrust): remove residual facade test copies #335 source layout.

Contributor Checklist

• Linked issue refactor(whatsrust): remove final facade-owned definitions #336.
• Added exactly one type:* label: type:refactor.
• Ran formatting and diff checks.
• Ran Rust workspace check and focused/API tests.
• Ran Go tests and vet; no Go files changed.
• Conventional commit format.
• No Co-Authored-By trailer.

Closes #336