Add Runtime fork and strand Lane contract

[flyingrobots]

Parent: #712

Outcome

Runtime.fork(source, { name }) creates a runtime-backed Lane whose kind is exactly strand, bound to the source Lane and its captured fork coordinate.

Acceptance

• Only a Lane owned by the same open Runtime can be forked.
• The returned descriptor is the mutually exclusive strand variant: kind, name, parent, and forkedAt.
• Fork construction is cross-lane Runtime behavior; no competing root factory is added.
• Strand write() and bounded observe() are runtime-honest and share the normal lifecycle/backpressure contract.
• Duplicate/invalid names, closed Runtime use, foreign Lanes, and unsupported source kinds fail with stable errors.
• Consumer, lifecycle, real-Git, root-boundary, and generated-reference tests agree.

Non-goals

Settlement, common-basis braid validation, and CLI/MCP commands are separate slices.

[coderabbitai]

🔗 Related PRs

#769 - feat(api): establish the v19 Runtime and Lane surface [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!