Add exports field for granular imports

[jbuckner]

Summary

Add an exports field to package.json enabling consumers to import specific parts of BookReader:

import '@internetarchive/bookreader'               // full shell (unchanged)
import '@internetarchive/bookreader/core'           // BookReader class only
import '@internetarchive/bookreader/jquery'          // jQuery wrapper
import '@internetarchive/bookreader/plugins/search/plugin.search.js'  // individual plugin
import '@internetarchive/bookreader/styles'          // BookReader.css

Problem

Currently, the only entry point is the full ia-bookreader shell. Consumers who want to use BookReader core with a custom shell, or who need to import jQuery/plugins separately for loading order control, have to use deep path imports that aren't guaranteed by the package contract.

Test plan

• Verify import '@internetarchive/bookreader' still works (default entry)
• Verify import '@internetarchive/bookreader/core' imports BookReader class
• Verify import '@internetarchive/bookreader/jquery' sets up jQuery globals

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.98%. Comparing base (2908b1e) to head (281a85a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1523   +/-   ##
=======================================
  Coverage   68.98%   68.98%           
=======================================
  Files          65       65           
  Lines        5556     5556           
  Branches     1229     1229           
=======================================
  Hits         3833     3833           
  Misses       1689     1689           
  Partials       34       34           

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[jbuckner]

Rebased onto current master (offshoot WEBDEV-8245 shipped today and is now using this patch stack downstream — re-ready for review when someone has a moment 🙏).