fix(tests): resolve qemu-img via PATH so the differential runs off macOS

[h4x0r]

The defect

The qemu-img oracle was pinned to the macOS-arm64 Homebrew path:

const QEMU_IMG: &str = "/opt/homebrew/bin/qemu-img";   // x2, lines 19 and 99

That path does not exist on the ubuntu-latest runner, so every qemu differential in this crate resolved nothing and returned early on every CI run — despite ci.yml installing qemu-utils in two separate jobs specifically to feed them. The tests reported 4 passed while validating nothing.

The fix

PATH first (covering any install location, including ones a fixed list cannot anticipate), the known absolute prefixes as fallback for a stripped PATH, QEMU_IMG_BIN as override. A candidate counts only if --version succeeds — proving the binary runs, which a bare Path::exists() does not.

This is the shape already used by sqlite-forensic's sqlite3_bin() (env + PATH + probe) merged with qcow2-core's candidate list, rather than a new invention.

Does the differential actually pass once the oracle is present?

Yes. Run on Linux (rust:1-slim-bookworm + apt qemu-utils 7.2.22), where this had never executed:

Control	Result
A — real qemu-img on PATH	✅ 4 passed, 0.20s
B — QEMU_IMG_BIN=/bin/true (resolves, exits 0, produces nothing)	4 FAILED
C — QEMU_IMG_BIN=/nonexistent	✅ 4 passed, 0.00s

Control B is the positive control: the tests must fail when the oracle produces no output, which proves the body genuinely runs the oracle and consumes its result. Control C's 0.00s is the fingerprint of the old bug — a green no-op, indistinguishable from real validation by the ok line alone.

VhdReader agrees with qemu-img byte-for-byte on Linux. No parser defect was hiding behind the skip.

On the missing RED commit

A test-plumbing change admits no honest failing test — a unit test asserting "the resolver does not hardcode one directory" would be tautological. The verification is the control matrix above, not a fabricated RED.

Gates

cargo build · cargo test --workspace · cargo clippy --all-targets -- -D warnings · cargo fmt --check — all green.

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