checklocks: enforce eventfd lock contracts

[tamird]

The event counter is protected by mu, and host I/O helpers require the same lock from their callers. Add the counter guard and replace the prose preconditions on the helpers with checklocks annotations so violations are detected during analysis.

Assisted-by: Codex

[tamird]

r? @fvoznika — could you review the event-counter guard and host-I/O helper preconditions, given your eventfd notification work?

eventfd_nogo and eventfd_test_nogo passed for Linux amd64 and arm64. The runtime eventfd_test could not build natively on macOS because of Linux-only dependencies, including unix.O_LARGEFILE; the same failure was reproduced on the unmodified base. Runtime validation remains for Linux CI.

Assisted-by: Codex

[tamird]

Closing in favor of consolidated #14343. The expanded replacement commits are prepared locally; the replacement branch has not been pushed yet.

Assisted-by: Codex