SourceGenerators: Emit BuildAsync for async assemble

[Skymly]

Summary

• When [BuilderAssemble] returns Task<T> or ValueTask<T>, emit type-state–gated BuildAsync(CancellationToken cancellationToken = default) instead of Build().
• Forward at most one assemble CancellationToken (excluded from step binding) and return the assemble task/value-task directly.
• Report DP086 for bare Task/ValueTask, duplicate assemble, and multiple CancellationToken parameters. Sync assemble still emits only Build().

Related Issue

Closes #326
Relates to #324
Follow-up docs: #327

Solution module

• SourceGenerators (DesignPatterns.SourceGenerators/)

Type of change

• Feature
• Source generator / diagnostic / CodeFix change

Test plan

• ./build.ps1 --target Ci --configuration Release (or CiPack if packaging changed)
• Sibling samples (if API/generator behavior changed): clone DesignPatterns.Samples beside this repo and run ./build.ps1 --target Ci — tracked as DesignPatterns.Samples#29, not blocking this PR

Breaking changes

• None

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
• 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 — tracked in Docs: ADR-011 + Step Builder Phase 2 async doc sync #327
• User-facing docs synced (separate PRs if multi-repo)
• CHANGELOG [Unreleased] updated for generated BuildAsync

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Stale comment

Autopilot: merge-ready.

• Mergeable (no conflicts); head is 1 commit ahead of main, 0 behind.
• Required CI green: build-test, build-test-net48, pack.
• No unresolved review threads or actionable comments.

Leaving merge to @Skymly.

  

_{Sent by Cursor Automation: DesignPatterns — Autopilot PR}

[cursor]

Autopilot: merge-ready.

• Mergeable (no conflicts); head is 1 commit ahead of main, 0 behind.
• Required CI green: build-test, build-test-net48, pack.
• No unresolved review threads or actionable comments.

Leaving merge to @Skymly.

  

_{Sent by Cursor Automation: DesignPatterns — Autopilot PR}