Docs: Step Builder Design Doc + ROADMAP/AGENTS/CHANGELOG

[Skymly]

Summary

• Add docs/design/StepBuilder.md covering attributes, generated {Holder}Builder type-state proof model, DP078–DP086, non-goals, and comparison vs registration/assembly *Builder types
• Record ADR-010 (generic type-state markers + required-step cap 8) and index it from the design doc / ADR README
• Update ROADMAP F3 Top-2 Builder status to shipped MVP [~], AGENTS pattern summary, and CHANGELOG [Unreleased]

Related Issue

Closes #291

Solution module

• Docs / Repository (README, docs/, .github/, AGENTS.md, build/)

Type of change

• Docs / repo metadata only

Test plan

• ./build.ps1 --target Ci --configuration Release
• Sibling samples (N/A — docs only)

Breaking changes

• None

Checklist

• This PR touches only one solution module (see AGENTS.md)
• Commit messages are in English
• PR description is English only
• 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)
• No documentation changes needed

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Note

Low Risk
Documentation and repository metadata only; no product code, APIs, or diagnostics behavior changes.

Overview
Documents the Step Builder domain for maintainers: new docs/design/StepBuilder.md covers [GenerateBuilder] / [BuilderStep] / [BuilderAssemble], the generated {Holder}Builder type-state model, DP078–DP086, MVP non-goals, and how Step Builder differs from registration-style *Builder types.

Adds ADR-010 (generic type-state for required-step completeness, cap of 8 required steps) and wires it into the design and ADR indexes. AGENTS.md, docs/ROADMAP.md (F3 Top-2 Builder → MVP [~], naming table), and CHANGELOG.md [Unreleased] are updated to match; no runtime, generator, or diagnostic code changes in this PR.

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