Phase 1b-i: Libc enum + Preprocessor trait + probe_libc resolver #500

Part of #493 Phase 1. Lands the fail-closed libc-probe scaffold: the Libc enum, the Preprocessor trait that abstracts "invoke a cross-compiler in -E mode", and the probe_libc function that parses the result into Libc::{Newlib, Picolibc, Unknown}.

The concrete ExternalPreprocessor (backed by std::process::Command) is intentionally deferred to a follow-up sub-issue so this PR stays small and fully unit-testable without needing a real cross-compiler at test time.

Scope

• Add crates/fbuild-build/src/shrink/probe.rs containing:
  • pub enum Libc { Newlib, Picolibc, Unknown } with Default = Unknown (fail-closed).
  • pub struct PreprocessResult { exit_code: Option<i32>, stderr: String }.
  • pub trait Preprocessor { fn preprocess(&self, source: &str) -> std::io::Result<PreprocessResult>; }.
  • pub fn probe_libc<P: Preprocessor>(p: &P) -> Libc that:
    • Submits a small TU with #error PICOLIBC and #error NEWLIB guards behind #ifdef __PICOLIBC__ and #ifdef __NEWLIB__ respectively.
    • Returns Picolibc if stderr contains "PICOLIBC".
    • Otherwise returns Newlib if stderr contains "NEWLIB".
    • Otherwise returns Unknown (the fail-closed branch — covers glibc / musl / probe errors / unknown toolchains).
• Wire pub mod probe; into shrink/mod.rs.

Out of scope

• The concrete ExternalPreprocessor impl backed by std::process::Command — separate follow-up sub-issue.
• Caching probe results per toolchain hash — separate follow-up sub-issue once ExternalPreprocessor lands.
• Wiring probe_libc into the auto-resolver — separate sub-issue (Phase 1c).

Pass

• cargo build --workspace --all-targets clean.
• cargo clippy --workspace --all-targets -- -D warnings exits 0.
• New unit tests cover all 5 paths:
  • mock returns picolibc-flavored stderr → Libc::Picolibc
  • mock returns newlib-flavored stderr → Libc::Newlib
  • mock returns clean (exit 0, empty stderr) → Libc::Unknown (glibc/musl case)
  • mock returns I/O error → Libc::Unknown (probe-failed case)
  • both markers present (defensive) → Picolibc wins (picolibc-on-newlib coexistence isn't a real config but the precedence is documented)