test: add table-driven tests for namespace resolve and spell

[Macawls]

Add table-driven tests for the exported Resolve and Spell functions in internal/namespace, which previously had no direct unit tests.

• TestResolve covers derived namespaces, named directives, core directives, test-file joining, and empty stems
• TestSpell covers core, named, derived, and no-stem cases for baseline key formatting

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.02%. Comparing base (e7084e7) to head (8372816).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #192   +/-   ##
=======================================
  Coverage   99.02%   99.02%           
=======================================
  Files          50       50           
  Lines        4214     4214           
=======================================
  Hits         4173     4173           
  Misses         41       41           

Flag	Coverage Δ
unittests	99.02% <ø> (ø)

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[mpyw]

Congrats and appreciate for the first contributor!