Harden length-changing position coverage

[ecochran76]

Summary

• add length-changing alignment coverage for matches at the beginning, middle, and end of one-glyph-per-line text objects
• expand public synthetic PDF smoke coverage across those positions for both left and right alignment
• add non-sensitive bbox assertion fields for the checked coordinate and measured delta

Validation

• python3 -m py_compile pdf_glyph_replace.py pdf_fixture.py pdf_inventory.py pdf_dogfood.py pdf_dogfood_summary.py pdf_mutation/*.py
• python3 -m unittest discover -s tests -v

---

^{Need help on this PR? Tag @codesmith with what you need. Autofix is disabled.}