SourceGenerators: GenerateBuilderGenerator + Verify

[Skymly]

Parent

#287

What to build

A source generator that turns a [GenerateBuilder] schema holder into {Holder}Builder: generic type-state markers for required steps (ADR-010), at-most-once step methods, gated Build() that invokes the [BuilderAssemble] method with name-bound arguments (optional unset → null). Emit diagnostics allocated in #289 for cap, missing assemble, binding mismatch, mutex, partial order, and related schema errors. Lock behaviour with Verify snapshots.

Acceptance criteria

• Generator emits {Holder}Builder with type-state markers; Build only on fully-set required markers (ADR-010)
• Assemble method discovered via [BuilderAssemble]; parameters bind by name; product type from return type
• Step methods are signature/schema only; state lives on the generated builder; steps at most once
• Optional steps do not consume required markers; unset optional args pass as null-compatible values
• Free order by default; After/Before and mutex groups reported as diagnostics (not type-state)
• Required-step count > 8 is a generator Error; optional steps excluded from the cap
• Verify covers happy path and diagnostic cases; no private-helper-only assertions
• Sync-only MVP; no DI/Autofac emission

Blocked by

• Runtime: GenerateBuilder attributes + type-state markers #288
• Diagnostics: allocate Step Builder diagnostic IDs #289