test: add adversarial edge coverage from the August PR review sweep

One file per merged PR (test_pr<N>_edges.py), written during review to
stress the changed behavior beyond each PR's own regression tests:
boundary inputs, malformed input, unicode, path spellings, scale, and
idempotency. All deterministic and network-free. 234 tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012fHfGDiZedoxjpKzanHri3