docs(calculate): add JSDoc for all exported functions and edge cases

[Ayushi-ninja]

Description

Fixes #1492

Pillar

• Other (Bug fix, refactoring, docs)

Visual Preview

N/A — this PR only adds JSDoc comments; no SVG output is changed.

Checklist before requesting a review:

• I have read the CONTRIBUTING.md file.
• I have tested these changes locally.
• I have run npm run format and npm run lint locally and resolved all errors.
• My commits follow the Conventional Commits format (docs(calculate): ...).
• I have not changed any README (no new theme or URL parameter added).
• I have starred the repo.
• I have made sure that I have only one commit to merge in this PR.
• The SVG output is unaffected — no rendering logic was changed.

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