Handle malformed coverage files

[ranocha]

Julia's .cov files can contain an empty or partial line (#70, #92). CoverageTools currently slices the first nine characters of every line, so one damaged file aborts coverage collection, even when another process wrote a usable coverage file.

This reads Julia's ASCII count field and treats malformed records as unknown counts. It keeps a slot for each bad record, so later counts stay on the right source lines and counts from another .cov file can still be merged. It warns once per affected file. Source exclusion also handles a coverage file that ends before the source file.

For example, counts of 1, an empty record, then 3 become [1, nothing, 3] rather than throwing or shifting 3 to the second line. Existing source analysis may still infer zero for an unknown line inside a function.

Tested with the full suite on Julia 1.10.12 and 1.14 nightly. The new tests cover empty and truncated records, count-field boundaries, merging with a complete file, and a coverage file shorter than its source.

Fixes #70. Fixes #92.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.08%. Comparing base (a0d2e8e) to head (b3b519b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   95.81%   96.08%   +0.27%     
==========================================
  Files           4        4              
  Lines         358      383      +25     
==========================================
+ Hits          343      368      +25     
  Misses         15       15              

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