refactor(whatsrust): extract callback registrations

[Andiveli]

Summary

• Extract public callback registration handlers from the whatsrust facade.
• Move the setup macro implementation into the callback registration seam and make its standard-library and trait paths hygienic for moved invocations.

Changes

• Added whatsrust/src/registrations.rs for callback and QR registrations.
• Kept CallbackTranslator in callbacks.rs and re-exported the public handlers from lib.rs.
• Preserved callback ownership, locking, ABI signatures, and process-lifetime allocation behavior.

Chain Context

• Start: PR refactor(whatsrust): extract event conversion #301 (refactor/whatsrust-event-conversion).
• End: callback handlers and macro isolated; later lifecycle, presence, media, and action slices remain.
• Dependency: this PR is the immediate parent for the lifecycle slice.
• Diagram: #301 -> 📍 #302 -> #303 -> #304 -> #305
• Deferred: Cargo compilation/tests are intentionally deferred per task instructions; only formatting/source/diff checks were run.
• Rollback: revert commit 578582c / remove registrations.rs and restore the handler block in lib.rs; no unrelated behavior changes.

Testing

• cargo fmt --check
• git diff --check
• Source and authored-line budget checks
• Cargo tests/build (deferred; no Cargo build/test command run)
• Manual runtime testing (not applicable to this mechanical extraction)

Closes #306