docs: write COMPARISONS.md — PDF Studio vs Adobe Acrobat vs Stirling-PDF

[Kayforkind]

Problem

People evaluating PDF tools search for comparisons. We have no honest, verifiable comparison page, so that traffic goes elsewhere.

Scope

• Create COMPARISONS.md at the repo root with a feature-comparison table: PDF Studio vs Adobe Acrobat vs Stirling-PDF.
• Cover at least 10 dimensions, e.g.: price, self-hosting, privacy model (where document bytes go), redaction honesty (visual whiteout vs true content removal), OCR, form filling, CLI, MCP/agent access, offline use, license.

Acceptance criteria

• Every table cell is verifiable: either tested by you or linked to a source. No marketing claims without evidence.
• The privacy/redaction rows are honest — never describe visual whiteout as secure redaction.
• Linked from the README (one line near the top is enough).