v19: add draft timeline previewJoin and join API

[flyingrobots]

Part of #712.

Problem

Speculative work should not require root users to manipulate strands and braids directly. v19 should expose drafts and joins while preserving formal WARP terms for advanced surfaces.

Target Shape

const draft = await timeline.draft('try-admin-role');
await draft.write(intent.property.set({ subject, key, value }));

const preview = await timeline.previewJoin(draft, {
  policy: 'deterministic',
});

if (preview.receipt.outcome === 'accepted') {
  await timeline.join(draft);
}

Scope

• Add timeline.draft(name) for speculative timelines.
• Add timeline.previewJoin(draft, options).
• Add timeline.join(draft, options).
• Keep Strand and Braid in advanced or legacy surfaces.

Acceptance

• Public docs use drafts and joins first.
• previewJoin() is a dedicated method, not a dryRun boolean trap.
• Join operations return receipts.
• Formal braid terminology remains available only where deliberately imported.