Runtime: GenerateBuilder attributes + type-state markers

[Skymly]

Parent

#287

What to build

Public Creational attributes and marker types that declare a Step Builder schema holder: [GenerateBuilder], [BuilderStep] (required / optional / mutex / After / Before metadata), [BuilderAssemble], plus the NotSet/Set (or equivalent) type-state marker types used by generated builders. Dual TFM, XML docs, no generator yet.

Acceptance criteria

• [GenerateBuilder] can mark a schema holder type
• [BuilderStep] can annotate step methods with required/optional, mutex group, and After/Before (or equivalent) metadata
• [BuilderAssemble] can mark the user assemble method
• Type-state marker types for required-step completeness ship in Core (usable by the upcoming generator)
• Public API has XML documentation; netstandard2.0 + net8.0 build clean under TreatWarningsAsErrors
• No source generator, diagnostics implementation, or DI surface in this ticket

Blocked by

• None — can start immediately.