[major] Make SignificantNumber a value type

[matt-edmondson]

Summary

SignificantNumber builds against ktsu.PreciseNumber 2.x again. PreciseNumber 2.0 became a readonly record struct, and a struct can't be inherited, so record SignificantNumber : PreciseNumber no longer compiles against it. SignificantNumber is now a readonly record struct that holds a PreciseNumber and implements INumber<SignificantNumber> itself. Generic math conversion (T.CreateChecked and friends) works, and default(SignificantNumber) is a valid zero.

This is a major version, because every consumer that relied on SignificantNumber being a class or a PreciseNumber is affected. docs/migration-guide-2.0.md covers each change.

Changes

• Composition instead of inheritance. The value lives in Value. A SignificantNumber converts to PreciseNumber implicitly, and the reverse is explicit because it opts into the significant figure rules. FromPreciseNumber and ToPreciseNumber() do the same as named methods.
• Members that used to be inherited are declared on the struct and delegate to Value with the same return types: Exponent, Significand, SignificantDigits, Abs(), Round, ReduceSignificance, Clamp, Squared, Cubed, To<T>, the ToString overloads, TryFormat, and CompareTo(object). NegativeOne, E, Pi, and Tau are now typed as SignificantNumber, and there are static Max, Min, Clamp, and Round.
• Conversions. TryConvertFrom* and TryConvertTo* delegate to PreciseNumber, and handle SignificantNumber and PreciseNumber first because PreciseNumber doesn't recognize SignificantNumber.
• Rule helpers (LowestDecimalDigits, LowestSignificantDigits, and the check for -1, 0, or 1) are private statics, because PreciseNumber 2.0 made its versions internal.
• Stack overflow removed. The explicit IUtf8SpanFormattable.TryFormat called itself. It's gone, and INumberBase's default implementation does the work.
• Package. ktsu.PreciseNumber is pinned to 2.0.3, which includes the conversion and rounding fixes from PreciseNumber chore: update ktsu.Sdk to 2.21.1 [patch] #72.

Breaking changes

• It's a struct, so it can't be null or derived from.
• The protected constructors and the CreateFromComponents overload that takes sanitize are removed.
• CompareTo(SignificantNumber?) is now CompareTo(SignificantNumber).
• The four TryParse overloads and TryConvertFrom* use out SignificantNumber, and TryParse yields zero instead of null on failure.
• Assigning a PreciseNumber to a SignificantNumber needs an explicit cast.
• ToSignificantNumber() returns a copy instead of the same instance.
• Static members inherited from PreciseNumber that aren't redeclared, such as MakeCommonized, are no longer available.

Testing

• 95 tests pass on net10.0, up from 75 on main. The 20 new ones in SignificantNumberValueTypeTests cover default, both conversions with PreciseNumber, CreateChecked, CreateSaturating, and CreateTruncating from generic code, a generic Sum<T>, the static helpers, the TryParse failure path, CompareTo(object), and UTF-8 formatting.
• Existing tests changed only where the value type forces it: IsNull on a failed parse became equality with Zero, AreSame became a value check, the unsanitized CreateFromComponents test is gone with the overload, and IsNotNull on Exp became a range check.
• ToSignificantNumberWithDigits_FromSignificantNumber_ReducesSignificance reduces 123.456 to three digits and expects 123. It fails on PreciseNumber 2.0.1, which added 55… instead of 50… when rounding, and passes on 2.0.3.

🤖 Generated with Claude Code

https://claude.ai/code/session_01K5Bk9UjGdGUtC5C6qK5ZxD

[sonarqubecloud]

Quality Gate failed

Failed conditions
63.2% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud