[T-066] The CLI accepts a non-PDF (e.g. an Office file) and emits a near-empty result instead of failing loud

[Ztein]

Canonical ticket: docs/tickets/T-066-cli-degrades-silently-on-non-pdf-input.md

figmark <file> (the CLI, main.run) passes the path straight to convert(), which opens it with PyMuPDF. Given an Office file, PyMuPDF opens it as a degraded near-empty "document" and the run completes with exit 0 — output that looks fine and is nearly empty, with no signal that anything went wrong. Exactly the failure class the project bans (fail loud, never silently degrade — T-024).