feat(fast_io): refuse SQPOLL when mmap reader is active

[oferchen]

Summary

• Adds a defensive configuration-time check on IoUringConfig::build_ring that disables IORING_SETUP_SQPOLL when the caller has signalled an mmap-backed basis reader is live on the same transfer plan. Closes Update Homebrew formula for v3.4.1-rust #2158.
• Pairing SQPOLL with a file-backed mmap region is a documented kernel hazard: the SQPOLL kthread services SQEs without the user mm context, so cold-page faults on mapped pages bounce through task_work on the original task (deadlock loop on pre-6.x kernels), and concurrent truncation surfaces as in-kernel SIGBUS. See docs/audits/io-uring-sqpoll-mmap-interaction.md plus companion audits mmap-iouring-sqpoll-pagefaults.md and io_uring_sqpoll_mmap_pagefault.md.
• The new IoUringConfig::mmap_basis_active field defaults to false. When true, build_ring emits a debug_log!(Io, 1, ...) warning, sets the existing SQPOLL_FALLBACK flag, and falls back to a regular ring rather than failing the transfer.
• Updates docs/audits/io-uring-sqpoll-mmap-interaction.md to mark the S1 caveat resolved with implementation reference.

Why this is defensive, not a bug fix

Today's wired paths already avoid the hazard: DeltaApplicator forces BufferedMap for the basis file when its writer is io_uring-backed, and no preset turns SQPOLL on. The check closes the audit's "OK-with-caveat" gap: a future caller that flips sqpoll = true alongside an MmapStrategy basis is now refused at config time, before any SQE reaches the kernel.

Test plan

• Unit test: default_config_mmap_basis_inactive confirms the field defaults to false across Default, for_large_files, for_small_files.
• Unit test: build_ring_sqpoll_with_small_files_no_mmap_keeps_request confirms SQPOLL stays enabled (or falls back for permission reasons) when no mmap basis is active.
• Unit test: build_ring_sqpoll_with_mmap_basis_disables_sqpoll confirms SQPOLL is refused and sqpoll_fell_back() returns true when mmap_basis_active: true.
• Unit test: build_ring_no_sqpoll_with_mmap_basis_no_warning confirms the flag is inert when SQPOLL was never requested.
• CI: fmt+clippy, nextest (stable), Windows, macOS, Linux musl.