v19: introduce openWarp and Timeline facade

[flyingrobots]

Part of #712.

Problem

openWarpWorldline() makes first-use application code start with internal history terminology. v19 should open a product-level Warp handle and then select timelines by name.

Target Shape

const warp = await openWarp({ storage, writer: 'agent-1' });
const timeline = await warp.timeline('events');

Scope

• Add openWarp() as the public opener.
• Add Warp and Timeline public handles.
• Preserve legacy access to openWarpWorldline() outside root.
• Keep implementation worldline/strand details behind the facade.

Acceptance

• Root quick-start can open a timeline without worldline language.
• Timeline supports the write/read/draft/join work planned by sibling v19 issues.
• openWarpWorldline() is not a root value export in v19.
• Legacy compatibility remains available from an explicit subpath.

[coderabbitai]

🔗 Related PRs

#107 - Prepare v18.0.0 release metadata and final gates [merged]
#110 - Worldline-first public API for v18 [merged]
#620 - Fix #282: prove architecture doc contract [merged]
#654 - Clean up versioned runtime names [closed]

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