refactor(whatsrust): extract presence wrappers

[Andiveli]

Summary

• Extract raw presence diagnostics, callback ingress counters, and presence subscription wrappers from the whatsrust facade.
• Preserve diagnostic ownership, counter reset behavior, ABI calls, and public result mapping.

Changes

• Added whatsrust/src/presence.rs.
• Re-exported SubscribePresenceResult, drain_raw_presence_diagnostics, and subscribe_presence from lib.rs.
• Kept callback registration behavior unchanged while routing ingress counting through the presence seam.

Chain Context

• Start: PR refactor(whatsrust): extract lifecycle wrappers #312 (refactor/whatsrust-lifecycle-wrappers).
• End: presence wrappers isolated; media and action slices remain.
• Dependency: this PR is the immediate parent for the media slice.
• Diagram: #301 -> #311 -> #312 -> 📍 #313 -> #314 -> #315.
• Deferred: Cargo compilation/tests are intentionally deferred per task instructions; only formatting/source/diff checks were run.
• Rollback: revert commit 9d43523 / remove presence.rs and restore the presence 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 #308