ci: run fuzz targets concurrently

[favonia]

Remove the matrix-level two-job cap so all three independent fuzz targets can run concurrently while each target remains limited to one fuzz worker via -parallel 1. This reduces fuzz-workflow latency without increasing per-target worker concurrency; the workflow passes YAML parsing and actionlint.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.49%. Comparing base (3a2ce73) to head (8fcfe80).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1246   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files         100      100           
  Lines        6249     6249           
=======================================
  Hits         6155     6155           
  Misses         81       81           
  Partials       13       13           

Flag	Coverage Δ
smoketests	11.88% <ø> (ø)
unittests	98.11% <ø> (ø)

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