Phase 1b-ii: ExternalPreprocessor (subprocess-backed libc probe) #502

Part of #493 Phase 1, follow-up to #500 / #501. Lands the concrete ExternalPreprocessor impl backed by std::process::Command that the auto-resolver will use to probe a real cross-compiler at build time.

Scope

• Add ExternalPreprocessor struct in crates/fbuild-build/src/shrink/probe.rs:
  • pub struct ExternalPreprocessor { compiler: PathBuf, extra_args: Vec<String> }
  • Constructor new(impl Into<PathBuf>) + builder with_args(impl IntoIterator<Item=impl Into<String>>).
• Implement Preprocessor for ExternalPreprocessor:
  • Invokes <compiler> <extra_args> -E -x c - with the probe source on stdin.
  • Discards stdout (Stdio::null), captures stderr.
  • Returns PreprocessResult { exit_code, stderr }.
  • Maps subprocess spawn / stdin write / wait_with_output failures to io::Error (the trait's fail-closed contract).

Out of scope

• Caching per-toolchain probe results — separate sub-issue once the auto-resolver lands.
• Auto-discovering the cross-compiler path from a Toolchain struct — that's part of Phase 1c (wiring probe_libc into the orchestrator).
• Anything else.

Pass

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings exits 0.
• Tests:
  • Constructor + builder shape (no subprocess).
  • Missing-compiler path → probe_libc returns Libc::Unknown (fail-closed verification end-to-end).
  • Host-cc smoke test (cc / gcc / clang if available) — verifies the subprocess wiring completes without an I/O error and returns a real exit code; does not constrain the libc kind (host could be glibc, musl, newlib-on-cygwin, etc.). Skipped with a clear message if no host C compiler is found.

Notes

• #[cfg(unix)] / #[cfg(windows)] should not be needed — Command handles platform differences and we use Stdio::null / Stdio::piped portably.
• The host-cc test is best-effort: GitHub Actions runners (ubuntu-latest, macos-latest, windows-latest with MSYS2/MinGW) all ship cc / gcc, so it will exercise the path in CI.