Fix sentence-case letterless bug, add regex timeout, tidy the generator

[matt-edmondson]

Fixes 9 more SonarCloud issues. Two are genuine defects rather than style.

Real defects

S2589 — IsSentenceCase rejected letterless values

FirstOrDefault over a string yields char, not char?, returning 0 when no letter is present. Assigning that to a char? made HasValue always true, so the guard the author wrote — only check the first letter if there is one — never fired:

char? firstLetter = value.FirstOrDefault(char.IsLetter);
if (firstLetter.HasValue && !char.IsUpper(firstLetter.Value))   // HasValue is always true

A value such as "123" therefore reached the uppercase check, char.IsUpper(0) returned false, and it was rejected — even though "" is accepted, which is the same "no case to check" situation.

⚠️ Behaviour change. "123" and "42 - 7 = 35" now validate as sentence case where they previously threw ArgumentException. That matches the written intent and the empty-string precedent, but it is a real change — say the word if you would rather keep letterless values invalid and I will instead make the rejection explicit.

A test pins the new behaviour.

S6444 — PatternValidationRule had no regex timeout

Both the pattern and the value being validated come from the caller, so a pathological combination could backtrack unboundedly (catastrophic backtracking / ReDoS). The Regex now carries a one-second match timeout, which surfaces as RegexMatchTimeoutException rather than hanging the caller.

Generator cleanup

Rule	Count	Change
S3923	2	The operator doc comment interpolated (op.Op == "*" ? "by" : "by") — the same value either way. Replaced with the literal.
S2325	2	EmitVectorType and EmitVectorOverloadType are now static.
S1172 + S107	3	Dropped the unused codeBlocker parameter from GenerateInner, and the unused typeName from AddUnitFactories — taking that method from 8 parameters to the 7 allowed.

Removing codeBlocker also revealed that one of GenerateInners two callers was allocating a CodeBlocker purely to pass it; that is gone too.

Generated output is byte-identical — both S3923 ternary branches already produced the same string, and the removed parameters were unused. Verified by zero drift under Semantics.Quantities/Generated/.

Verification

• dotnet build — 0 errors, 0 warnings, all target frameworks
• dotnet test — 1089/1089 pass
• No generated-file drift

Remaining after this — ~57

S3776 cognitive complexity ×9 (mostly QuantitiesGenerator, now at 84% coverage so safer to refactor), S1133 deprecation TODOs ×10, S1192 ×15 in the generators, S4136 ×8 in SemanticString.cs, S1172 ×4 (the remaining unused generator parameters), S3267 ×6, S2342 ×2, S4144 ×1.

[sonarqubecloud]

Quality Gate failed

Failed conditions
50.0% Coverage on New Code (required ≥ 80%)
5.4% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud