Add %lsmagic -v for magic load debugging

[blink1073]

Closes #193.

Summary

• reload_magics() now records magic_search_paths and magic_load_errors on the kernel after each load cycle
• %lsmagic gains a -v/--verbose flag that prints the search paths and any load errors (or "No load errors.")
• Adds a "Debugging magic loading" section to docs/new_magic.md explaining how to use %lsmagic -v and how to diagnose the editable-vs-regular-install issue described in the issue

Test plan

• %lsmagic output unchanged without -v
• %lsmagic -v shows search paths and "No load errors." on a clean kernel
• A broken magic file produces an entry under "Load errors:" when using -v
• All existing tests pass (just test)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.97%. Comparing base (e203cf8) to head (c1e935b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #366      +/-   ##
==========================================
+ Coverage   90.92%   90.97%   +0.05%     
==========================================
  Files          51       51              
  Lines        2886     2903      +17     
  Branches      404      408       +4     
==========================================
+ Hits         2624     2641      +17     
  Misses        183      183              
  Partials       79       79              

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