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

[Skymly]

Summary

Record Step Builder Phase 2 async assemble as ADR-011 and sync maintainer docs with the BuildAsync exit already shipped in #326 / #329.

ADR-010 remains the type-state decision and is not superseded. Async is assemble-exit only: sync T still emits Build(); BCL Task<T> / ValueTask<T> emit only BuildAsync. Product type is the inner T. Cancellation-token rules and the tightened DP086 contract are documented. ROADMAP no longer lists async as an open Phase 2 item; DI and step validation remain.

Related Issue

Closes #327

Relates to #324

Solution module

• Runtime (DesignPatterns/)
• Diagnostics (DesignPatterns.Diagnostics/)
• SourceGenerators (DesignPatterns.SourceGenerators/)
• Analyzers (DesignPatterns.Analyzers/ + DesignPatterns.CodeFixes/)
• DependencyInjection (DesignPatterns.Extensions.DependencyInjection/)
• Package (DesignPatterns.Package/)
• Docs / Repository (README, docs/, .github/, AGENTS.md, build/)

Type of change

• Bug fix
• Feature
• Source generator / diagnostic / CodeFix change
• Refactor (no behavior change)
• Docs / repo metadata only

Test plan

• Checked ADR-011 and docs/design/StepBuilder.md against the merged generator behavior in SourceGenerators: Emit BuildAsync for async assemble #329: exclusive exit, direct task return, optional CancellationToken forwarding, DP086 illegal contracts
• ./build.ps1 --target Ci --configuration Release — not run; docs and registry text only, no code or packaging change
• Sibling samples — not in this PR. Async sample remains DesignPatterns.Samples#29

Breaking changes

• None
• Yes — describe migration steps (APIs may still be pre-stable):

Checklist

• This PR touches only one solution module (see AGENTS.md)
• Commit messages are in English (no AI/agent tooling mentions in commits)
• PR description is English only — no AI/agent/Cursor tool attribution, no auto-generated summary blocks (e.g. CURSOR_SUMMARY, "Made with …")
• No version bumps, tags, releases, or NuGet publish steps included unless explicitly requested
• Public API / diagnostic / generated code changes are documented if user-visible

Documentation checklist

• Design Doc updated if API / diagnostic / implementation changed
• User-facing docs synced (separate PRs if multi-repo) — maintainer docs only; user site is out of scope for Docs: ADR-011 + Step Builder Phase 2 async doc sync #327
• No documentation changes needed

[Skymly]

Auto review (docs-only)

Overall this is a coherent maintainer-doc sync for Step Builder Phase 2 async: ADR-011, Design Doc, ROADMAP, indexes, CHANGELOG, and the DP086 registry line all tell the same story (exclusive Build / BuildAsync exit, product type = inner T, CT forwarding rules, no new diagnostic IDs). Scope matches the Docs module checkbox and the #325 → #326 → #327 slice order.

Findings

1. ADR-010 vs ADR-011 process tension (should fix or explicitly decide) — docs/DOCUMENTATION.md §2 says Accepted ADR bodies are immutable and later changes go through Superseded by ADR-NNN. ADR-010 still states (decision §1/§5/§6) that only Build() is gated, product type = assemble return type, and async Build/assemble is an MVP non-goal. ADR-011 says it does not supersede ADR-010 while also “lifting” §6 and narrowing §5 for async. A reader who opens ADR-010 alone still gets the old contract. Prefer one of:

   • mark ADR-010’s status line as partially superseded for the async / product-type bullets (if status metadata is allowed to change), or
   • have ADR-011 explicitly supersede those bullets and restate that type-state / the required-step cap remain from ADR-010.
     Leaving both as Accepted with contradictory async wording is the main doc risk in this PR.

2. AGENTS.md DP080 row not updated — DP086’s registry line was tightened for Task<T> / ValueTask<T> / bare task / multiple CancellationToken, but DP080 still reads only as “assemble 参数无法绑定到步骤”. docs/design/StepBuilder.md already notes the async CancellationToken exemption; the AGENTS diagnostic table should match so the registry stays the single source of truth.

3. Nit (non-blocking) — Design Doc header version stays v0.2.4-preview1 while Phase 2 async content lands. Fine if you intentionally tie the doc stamp to the package train; otherwise a small doc revision note would make the Phase 2 sync obvious.

Looks good

• ADR index next number → ADR-012; design index links both ADR-010 and ADR-011.
• ROADMAP Top-2 and “已完成” bullets correctly move async out of open Phase 2 while leaving DI / step validation and Samples#29 open.
• WorkGraph comparison row updated for BuildAsync.
• Negative consequences in ADR-011 (split holder for dual exits; custom awaitable stays sync-shaped; token accepted but maybe not observed) are documented honestly.

No code or packaging changes; skipping CI for docs-only is reasonable. Not a merge blocker if you resolve (1) deliberately; (2) is a small follow-up in this PR or a tiny docs fix.

[Skymly]

This paragraph both keeps ADR-010 as the type-state decision and “lifts” its §6 async non-goal / narrows §5 product-type wording. Under docs/DOCUMENTATION.md §2, Accepted ADR bodies are immutable and later changes are supposed to go through Superseded by. As written, ADR-010 §1/§5/§6 still contradict this page for anyone who does not also open ADR-011.

Please either (a) mark ADR-010’s status as partially superseded for those bullets, or (b) state here explicitly which ADR-010 bullets this ADR supersedes while type-state / the required-step cap remain. Silent coexistence of two Accepted ADRs with opposite async guidance is the main consistency risk.

[Skymly]

DP086 is updated for the Phase 2 assemble contract, but the DP080 row a few lines above still omits the async CancellationToken exemption that ADR-011 and docs/design/StepBuilder.md now document. Please align the DP080 registry wording (token skipped for step-name binding on async assemble) so AGENTS stays the single diagnostic registry.