fix(comm): report mark-read outcome status

[ohdearquant]

Summary

• add explicit success / failed status to each mark-read result
• classify bulk outcomes as success, partial, or failed from their aggregate counts
• document the result statuses in comm discovery and user guidance

Verification

• lock_contention_reports_failed_mark_read_statuses holds a real SQLite writer lock and verifies honest singleton and zero-mark bulk failure results
• bulk_read_response_reports_success_partial_and_failed_statuses covers all aggregate status states
• cargo fmt --all
• cargo clippy -p khive-pack-comm --all-targets -- -D warnings
• cargo test -p khive-pack-comm (371 passed)

Fixes #1939
Fixes #1888