fix(wasix): complete built-in subprocesses and support version flags

[theduke]

What and why

A WASIX guest running Command::new("wasmer").arg("--version").output() printed wasmer run usage and then waited forever.

Two independent problems caused this:

• The built-in wasmer command did not recognize -V or --version.
• proc_spawn3 and legacy proc_spawn discarded the built-in task handle, so its completion never reached the forked child process observed by waitpid/proc_join.

Fix

The built-in now prints wasmer <version> to its child stdout, honoring pipe and file redirections used by Command::output(). Spawned virtual-command completion is relayed to the child process for both current and legacy spawn paths, including asynchronously completing commands.

Regression coverage

A C WASIX fixture spawns wasmer --version with child stdout/stderr pipes, reads the captured output, and waits for exit. Before the fix it timed out; it now verifies version output on stdout, empty stderr, and exit status 0 in both the legacy and current ABI configurations.

Validated with focused Cranelift tests for both configurations, cargo fmt, and cargo clippy -p wasmer-wasix --lib --features sys-thread -- -D warnings.

Fixes #6873
Fixes #6874

Linear: RUN-1088, RUN-1089

[Copilot]

Pull request overview

This PR fixes a WASIX subprocess hang when spawning built-in commands (notably wasmer --version) by (1) adding -V/--version handling to the built-in wasmer virtual command and (2) propagating completion of built-in virtual-command tasks to the spawned child process so waitpid/proc_join can observe termination. It also adds a regression test that spawns wasmer --version, captures stdout/stderr via pipes, and waits for exit.

Changes:

• Relay built-in virtual-command task completion to the forked/spawned child process status in both proc_spawn3 and legacy proc_spawn.
• Add -V/--version support to the WASIX built-in wasmer command and write version output to child stdout (respecting redirection).
• Add a WASIX C fixture regression test that spawns wasmer --version, reads stdout/stderr pipes, and asserts exit/status/output.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/wasix/tests/wasm_tests/process/builtin-wasmer-version/main.c	New regression fixture that spawns wasmer --version, captures output via pipes, and waits for termination.
lib/wasix/src/syscalls/wasix/proc_spawn3.rs	Propagates built-in virtual-command completion to the spawned child process in the WASIX ABI v3 spawn path.
lib/wasix/src/syscalls/wasix/proc_spawn.rs	Propagates built-in virtual-command completion to the spawned child process in the legacy spawn path.
lib/wasix/src/syscalls/wasix/mod.rs	Adds the shared propagate_virtual_task_completion() helper used by both spawn paths.
lib/wasix/src/os/command/builtins/cmd_wasmer.rs	Adds -V/--version flag support and writes version output to child stdout honoring redirection/pipes.