feat: discriminatedUnion and reworked oneOf errors

[joanllenas]

Summary

• Add discriminatedUnion decoder for tagged unions - validates only the matching variant and reports a precise error on unknown tags, avoiding the noise of oneOf trying every branch.
• Rework oneOf failure output: replace the old "deepest path wins" heuristic with a no alternative matched (tried N) summary followed by every branch's failure; same-path issues collapse into X or Y messages.
• Add formatIssuePath helper and use it in parse()/decodePromise() so array indices render with bracket notation (items[0] instead of items.0).
• Fix enum error format to use JSON.stringify consistently with other primitive decoders.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.05%. Comparing base (3223366) to head (0201324).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   98.65%   99.05%   +0.40%     
==========================================
  Files          25       26       +1     
  Lines         297      317      +20     
  Branches       88       93       +5     
==========================================
+ Hits          293      314      +21     
+ Misses          4        3       -1     

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