Check the public surface survives hostile input

[twcclegg]

The ported tests cover valid input, matching the Java originals. Nothing covers hostile input — and
this session found two crash bugs of that shape by accident: an unbounded stackalloc in the public
Normalize* methods (uncatchable StackOverflowException), and KeyNotFoundException geocoding an
Ascension Island number.

~30 inputs (lone surrogates, astral pairs, RTL overrides, NUL, non-ASCII digit scripts, characters
whose numeric value isn't a decimal digit, malformed RFC3966, 10k-char strings) go through Parse,
the normalizers, IsViablePhoneNumber, ExtractPossibleNumber, FindNumbers and
AsYouTypeFormatter, asserting nothing escapes but NumberParseException. Plus odd region codes and
a guard for the 200k-char case.

EveryRegionSurvivesTheReadOnlySurface runs geocoder, carrier, timezone and short-number lookups over
an example number from every region — the shape that would have caught the AC bug.

[Fact] over an array rather than [Theory]/[MemberData], because xunit serialises theory
arguments for discovery and lone surrogates don't survive it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.01%. Comparing base (ab514e1) to head (b5e7e67).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #379   +/-   ##
=======================================
  Coverage   98.01%   98.01%           
=======================================
  Files          39       39           
  Lines       52818    52818           
  Branches     1105     1105           
=======================================
  Hits        51770    51770           
  Misses        795      795           
  Partials      253      253           

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