feat: distinguish disabled vs failed registration in RegisteredBufferStats

[oferchen]

Summary

• Add RegisteredBufferStatus { Enabled | Disabled | RegistrationFailed { reason } } so observability can tell why a transfer is using non-registered buffers.
• Route IoUringReader / IoUringWriter construction through a new RegisteredBufferGroup::try_new_with_status helper that captures the kernel's formatted errno on failure, and expose the value via registered_buffer_status().
• Mirror the enum on the non-Linux stub so cross-platform telemetry surfaces stay cfg-free.
• Mark the IU2 row in docs/audits/io-uring-fixed-buffer-invariants-audit.md as RESOLVED.

Previously, Option<RegisteredBufferGroup> collapsed two operator-visible cases - caller-opted-out via IoUringConfig::register_buffers = false, and kernel-rejected registration (low RLIMIT_MEMLOCK, IORING_MAX_REG_BUFFERS exceeded, seccomp, etc.) - into the same None, leaving no signal to diagnose why the unfixed IORING_OP_READ / IORING_OP_WRITE path was in use. The new enum closes that observability gap.

Refs #2156. Sibling design notes in docs/audits/iouring-fixed-buffer-registration.md Section 5a.

Test plan

• try_new_with_status_disabled_when_flag_off - confirms enabled=false returns Disabled, never RegistrationFailed.
• try_new_with_status_enabled_on_success - confirms successful registration yields Enabled and a live group.
• try_new_with_status_registration_failed_carries_reason - forces failure via the wrapper's MAX_REGISTERED_BUFFERS ceiling and confirms the formatted errno reaches the caller.
• with_ring_skips_registration_when_disabled - extended to assert the writer's status is Disabled.
• registered_buffer_status_failed_when_kernel_rejects - confirms the writer's status is RegistrationFailed when registration is requested but rejected.
• CI: fmt+clippy, nextest (stable), Windows, macOS, Linux musl all green.