fix: correct Token.Position.Offset when YAML contains comments

[Yanhu007]

Fixes #856

Problem

Token.Position.Offset is incorrect when YAML contains comments. Each preceding comment shifts subsequent token offsets by -1:

tokens := lexer.Tokenize("# comment1\n# comment2\nkey: value")
// "key" token:
//   Offset: 21 (actual)
//   Offset: 23 (expected, 1-indexed)
//   Shift: -1 per comment = -2

Root Cause

In scanComment, the # character is consumed with s.progress(ctx, 1) which advances the source index (ctx.idx) but does not update s.offset. This creates a cumulative offset drift of -1 per comment.

Fix

Replace s.progress(ctx, 1) with s.progressOnly(ctx, 1), which advances both the source index and the offset counter.

All existing tests pass.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.65%. Comparing base (edee2f9) to head (e2cb4e1).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #867   +/-   ##
=======================================
  Coverage   80.65%   80.65%           
=======================================
  Files          22       22           
  Lines        6845     6845           
=======================================
  Hits         5521     5521           
  Misses        905      905           
  Partials      419      419           

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