Fix off-by-one in column numbers when applying source maps

[rmuratov]

Closes #22
Closes #26

What

Browsers report stack trace columns as 1-based, while the source-map library expects 0-based columns on lookup and returns 0-based columns in results. Both directions were passed through unconverted, so lookups could resolve to a neighboring mapping and displayed columns were off by one.

Changes in src/lib.ts

• tryGetOriginalPosition now passes column - 1 (clamped to 0) into originalPositionFor — the library throws on negative columns, and some tools emit 0-based columns in traces.
• Mapped positions are converted back to 1-based columns for display.
• The !column guard is replaced with column == null, so column 0 frames are transformed instead of being silently skipped (column === 0 is skipped due to falsy check in tryGetOriginalPosition #26).
• When the lookup finds nothing, originalPositionFor returns an all-null position; previously that was rendered as at (null:null:null). Such frames now fall back to the original line.

Tests

• New column numbers test suite drives the full App with a crafted source map (built via SourceMapGenerator) where 0-based columns 10 and 11 map to different original lines, so the off-by-one produces a visibly wrong result. Covers the conversion, the column-0 case, and the unmapped-position fallback. All three tests fail against the old lib.ts and pass with the fix.
• regular fixture expectations updated (+1 on mapped columns). Verified against the embedded sourcesContent: the new 1-based columns point exactly at the start of the call expressions (previousInstance(event), processDispatchQueue(...), fn(a), …), where the old values pointed at the whitespace before them.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (6c700e3) to head (364f614).

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #35   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            9         9           
  Lines          203       203           
  Branches        49        51    +2     
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  Hits           203       203           

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