fix(visitor): record string occurrence for accurate position reporting

[TobyTheHutt]

Fix string position tracking so duplicate string literals report all occurrences correctly.

Previously, addString only stored positions for the first occurrence and when minOccurrences was reached, which could under-report locations. The visitor now records every occurrence while still using the existing count map for threshold checks.

Changes:

• Updated visitor.go to append a position for every matching string occurrence.
• Unit coverage in visitor_test.go with full position capture across different minOccurrences values.
• Integration coverage in integration_test.go with end-to-end position counts from testdata.

Resolves: #45

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff            @@
##             master      #49   +/-   ##
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  Coverage          ?   66.75%           
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  Files             ?        4           
  Lines             ?      764           
  Branches          ?        0           
=========================================
  Hits              ?      510           
  Misses            ?      222           
  Partials          ?       32           

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