Function attest 

pub fn attest(
    root: &Path,
    arch: &str,
    compiler: &str,
    flags: &BTreeMap<String, Vec<String>>,
) -> Attestation

2026-09-26: Compute an attestation for every target from the working tree.

A record carries instead the attestation baked into the measuring binary (metrale_kernels::TARGET_CLOSURES, attached by GateRecord::with_closure), because the tree and the binary can differ. This tree-side version builds test fixtures and is what crates/server/tests/closure_attestation.rs compares the baked values against. A target whose sources do not resolve, or whose hash fails, is omitted.