Tests: a leaner differential for the character-range scan

[mathieu17g]

The differential test that #148 added compares the scan with the loop it replaced on every code point, on short byte sequences and on placed cases. It threw about 100,000 exceptions per run, and a throw costs a backtrace: about 10 s of the suite on my machine, and about 30 s on the Linux runners (152 s to 184 s). The macOS aarch64 job varies from 109 s to 411 s from one run to the next, with or without these tests, so its time says nothing here.

The reference loop now returns its verdict instead of throwing it, with Base.ismalformed and Base.isoverlong as the test UInt32(c) applies before it throws; the four-byte sequences come from an eight-byte alphabet and the placed cases from the twenty single bytes and the eighteen curated cases. Every code point, both SubString paths and the StringView case stay. The block runs in 4.5 s instead of 18 s on my machine, and the suite in 54 s instead of 72 s; the Linux jobs are back to 139 s and 97 s.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (a78327e) to head (103f8ef).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #151   +/-   ##
=======================================
  Coverage   96.67%   96.67%           
=======================================
  Files          15       15           
  Lines        2649     2649           
=======================================
  Hits         2561     2561           
  Misses         88       88           

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