fix(generator): resolve expr-based headings to the target filename

[toiroakr]

Summary

• generate scaffolds an expr-based heading (e.g. filename == heading) as that literal expression text, since the generator doesn't know the target filename — the file then fails its own check.
• Now tries the filename itself as the heading, verifies it against the real expression (vast.EvaluateHeadingExpr), and only falls back to the old text if that candidate doesn't actually satisfy it.

$ mdschema generate --schema schema.yml --output CreateOrder.md
$ head -1 CreateOrder.md
# filename == heading      # before
# CreateOrder              # after

[jackchuka]

Thanks!