FD corruption experiments

[theduke]

• fix stdio fd_close ordering race
• fix stale fd cleanup removals
• fix stale fd cleanup removals

[Copilot]

Pull request overview

This PR addresses file-descriptor (FD) reuse races during close/cleanup in WASIX by ensuring flush operations operate on captured file handles rather than fd numbers that may be reallocated concurrently.

Changes:

• Refactors fd_close to close the FD first while retaining a flush handle, preventing async flush/close from impacting a newly-reused fd number.
• Updates WasiFs::close_all and WasiFs::close_cloexec_fds to snapshot targets, remove them from the fd map under a write lock, then flush via captured handles to avoid stale cleanup removing newly-created/reused FDs.
• Adds regression tests covering fd-slot reuse and concurrent descriptor creation during cleanup.

Reviewed changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated 1 comment.

File	Description
lib/wasix/src/syscalls/wasi/fd_close.rs	Introduces close_fd_and_prepare_flush and adds a regression test for stdio fd reuse during delayed flush.
lib/wasix/src/fs/mod.rs	Reworks cleanup routines to remove FDs before flushing via captured targets; adds tests for concurrent creation and fd-slot reuse during cleanup.