docs: backlog research — leaner root plugin file

[dknauss]

Captures a GSD backlog research item (no code change): evaluate the performance, security, and maintainability benefits of refactoring the monolithic main plugin file (bibliography-builder.php) into a leaner bootstrap + includes/ modules.

Context: a half-finished extract-to-includes/ refactor already exists in the tree — includes/pmid.php is a dead, never-required duplicate of functions still defined inline in bibliography-builder.php (a latent redeclare-fatal). That near-miss motivates evaluating the refactor properly. The dead-file removal is noted as a prerequisite cleanup, tracked separately.

Adds: .planning/todos/pending/2026-06-17-research-benefits-of-a-leaner-root-plugin-file.md.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.47%. Comparing base (710b7aa) to head (052975e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #38   +/-   ##
=======================================
  Coverage   81.47%   81.47%           
=======================================
  Files          40       40           
  Lines        2613     2613           
  Branches      464      464           
=======================================
  Hits         2129     2129           
  Misses        126      126           
  Partials      358      358           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.