Generic sweeps call the span overload when they mean Parse(string): ISpanParsable hides IParsable in overload resolution #142

Found while writing the sibling parity sweeps (#139).

Problem

In generic code constrained on IRangeFactory<TRange, T> or IValueSetFactory<TSet, T>, a call such as TSet.Parse(text, provider) or TSet.TryParse(text, provider, out var r) with a string argument binds to the span overload. Both interfaces derive from ISpanParsable<TSelf>. Overload resolution removes the base interface's IParsable<TSelf>.Parse(string, …) candidate whenever the derived interface's Parse(ReadOnlySpan<char>, …) is applicable, and the implicit string → ReadOnlySpan<char> conversion makes it applicable.

Measured: with Int32Set.TryParse(string) seeded to read a blank literal as {}, a generic TSet.TryParse(" ", …) under an IValueSetFactory constraint still answers false. The same call under an IParsable<TSet> constraint answers true.

So the generic convention sweeps that say they cover Parse/TryParse "through every entry point" never call the string overloads. Each type declares those overloads separately (SetFormat.Parse(s.AsSpan(), …)), and they're what reflection-based consumers find, such as minimal-API binding's TryParse lookup. Today every pair delegates to the same parser, so nothing is wrong yet. But a string overload can drift without any sweep noticing.

Call sites (generic string calls under such a constraint):

• DecorativeElementContractTests
• LiteralProviderContractTests (3)
• RangeLiteralGrammarContractTests (3)
• SetElementWhiteSpaceContractTests (4)
• TemporalLiteralStrictnessContractTests (8)
• WrapperGrammarContractTests (3)
• ShapeMatrixParityTests (integration, 1)

Change

Route those calls through StringOverloads.Parse<T> / StringOverloads.TryParse<T> (added in #139, constrained on IParsable<TSelf> alone). Where a sweep lists Parse(span) as a separate entry point, it then really compares the two. For the oracles among them, show the reach with the set-tryparse-reads-blank-as-empty seed or a string-only sibling.

Done

• No generic Parse/TryParse call with a string argument goes through an ISpanParsable-derived constraint (a convention test could hold this, in the style of EngineDispatchConventionTests)
• A seed that changes only a string overload is caught by the literal sweeps that claim it