fix(cli): resolve test imports from the project root

[slepp]

Summary

hew test compiled each harness from a staged copy in a temporary directory, so the compiler inferred that directory as the project root and no cross-file import resolved inside any test. Harnesses now compile in memory against the original test path with the project root threaded through, making test-time import resolution identical to hew run. A multi-file fixture and a string-path regression pin the behaviour.

Verification

• cargo test -p hew-cli --test test_runner_e2e: 30 passed, 0 failed
• cargo test -p hew-cli --bin hew test_runner: 27 passed, 0 failed
• Full CLI preflight: pass
• Compiled ratchet: 0 new failures
• Preflight: ready-to-push

Out of scope

• Test-runner discovery rules and layout conventions are unchanged; only where harnesses compile from is different.
• Package-level import surface for *_test.hew peers is handled separately and untouched here.

[gertybotbot]

CI is red because the new hew-cli/tests/test_project_root_fixture/tests/project_root_test.hew is swept by make hew-check-all outside hew test; its standalone check starts at tests/, so test_project_root::answer cannot resolve. The e2e test covers the hew test path, but please either make the fixture independently checkable or exclude/classify it in the corpus sweep.

[slepp]

The red check is deterministic: hew-check-all compiles project_root_test.hew standalone from its tests/ directory, so test_project_root::answer cannot resolve. Current main still reproduces the underlying hew test project-root failure, so the feature is not superseded, but this branch now conflicts with #2958. The fix needs a rebase that preserves the package-FFI test path plus a corpus-safe fixture arrangement.