feat: sequence diagrams

[hyamero]

Second of three PRs toward the archify gallery. This one covers the two sequence examples, Cache miss request and Async job roundtrip. It is stacked on #17: merge #17 first, then retarget this PR to main before merging.

The model. kind: "sequence" reuses the one diagram shape:

• Nodes are the participants.
• Edges are the messages, in array order: kind sync, async or return, plus tone.
• phases[].edges are time bands.

The agent gives only order. archify makes it author a pixel y for every message and every activation bar.

Layout (packages/layout/src/sequence.ts):

• Lifeline spacing comes from the labels, and each message gets its own row.
• Activation bars come from a per-participant stack:
  • a call opens a bar; a callback to a participant waiting on its own call nests one;
  • a reply closes the bar opened by the call it answers;
  • an unanswered bar ends where its participant was last busy.
• 2000 messages lay out in about 20ms.

Validation. Warnings for a reply that answers no call and for a band that covers messages it doesn't list. Overlapping bands are an error.

Viewer:

• Lifelines, activation bars, time bands and text labels.
• The intro plays in time order.
• Fit and export frames include edge labels.

Docs. The skill teaches sequences, including a Mermaid sequenceDiagram mapping.

Reviewed in a fresh context together with the next PR. The findings for this part (callback bars, a clipped self-call label, band gaps, docs) are fixed with tests.

[github-actions]

🎉 This PR is included in version 0.1.0-rc.1 🎉

The release is available on:

• v0.1.0-rc.1
• GitHub release

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[github-actions]

🎉 This PR is included in version 0.1.0 🎉

The release is available on:

• v0.1.0
• GitHub release

Your semantic-release bot 📦🚀