fix: xtask conformance tests fail — binary path resolution broken

[EffortlessSteven]

Problem

3 xtask tests fail, blocking CI and PR #5 merge:

• tests::run_with_args_dispatches_conform_quick — conformance tests 3 (Survivability) and 9 (Tool error code) can't find the diffguard binary
• tests::run_with_args_dispatches_ci_with_fake_cargo — cascading failure from mutex poisoning
• tests::run_with_args_dispatches_mutants_with_fake_cargo — cascading failure

Root Cause

cargo_bin_path() in xtask/src/conform_real.rs resolves to the xtask test binary instead of the diffguard binary when running cargo test -p xtask.

Fix

See .hermes/plans/001-conformance-test-fix.md for full plan:

1. Fix cargo_bin_path() to fall back to workspace-relative target/debug/diffguard
2. Build the binary if missing
3. Validate the path points to the correct binary (not xtask)

Acceptance Criteria

• All 15 conformance tests pass in quick mode
• cargo test --workspace exits 0
• CI pipeline green on PR feat: v0.2 enhancements — LSP, multi-base diffs, directory overrides, analytics #5