Docs: ADR-011 + Step Builder Phase 2 async doc sync

[Skymly]

Parent

#324

What to build

Record Step Builder Phase 2 async assemble decisions as ADR-011 (do not supersede ADR-010) and sync maintainer docs so Design Doc / ROADMAP reflect delivered BuildAsync behavior and remaining non-goals.

Acceptance criteria

• ADR-011 accepted: async assemble exclusivity, Task<T>/ValueTask<T>, product type = inner T, CT rules, DP086 extension; ADR index lists ADR-011; next number advances
• ADR-010 remains the type-state decision; ADR-011 notes it lifts the MVP async non-goal
• docs/design/StepBuilder.md documents async assemble / BuildAsync and updated limitations
• ROADMAP (and AGENTS summary only if needed) reflects Phase 2 async status
• Single-module PR: Docs only; Closes this issue; Relates to Spec: Step Builder async assemble / BuildAsync (Phase 2) #324

Blocked by

• SourceGenerators: Emit BuildAsync for async assemble + Verify #326