docs(sparkline): add Compared to alternatives section

[ob-aion]

Summary

• Adds a ## Compared to alternatives section to the README, mirroring the pattern in @coroboros/clone.
• Compares @coroboros/sparkline against sparkline (shiwano, 108k weekly downloads), node-sparkline, @fnando/sparkline, and react-sparklines (222k weekly downloads).
• Table rows are feature-presence only (output format, Node-friendliness, deps, a11y, precision, types, dual build) — no performance numbers, since there's no shared methodology.
• Positioning paragraph names the gap (accessibility-aware SVG strings rendered in Node) and the closest peer (node-sparkline).
• TOC entry added between API and Contributing.

Test plan

• Render the README on github.com — anchor #compared-to-alternatives resolves from the TOC.
• Table renders with right-aligned status columns in GitHub-flavored markdown.
• No regressions in existing sections (API, Gallery, Errors, Debugging).

[ob-aion]

OK