docs(constructive-sdk-events): add EventReferral max_depth documentation

[pyramation]

Summary

Documents the new max_depth parameter on EventReferral that enables multi-level MLM referral chains. This was implemented in:

• Upstream: feat(node-type-registry): add max_depth parameter to EventReferral constructive#1201 (node-type-registry)
• Generator + tests: https://github.com/constructive-io/constructive-db/pull/1261

Changes:

1. SKILL.md — Added a full EventReferral Blueprint Node section with:

   • Blueprint JSON example showing max_depth: 5
   • Config reference table (all 7 parameters including max_depth)
   • Toggles & Controls section (build-time, runtime, scope constraint)
   • "How Multi-Level Works" explanation with pseudocode
   • Updated description with max_depth, multi-level referral, MLM, referral chain trigger phrases

2. references/event-referral.md (new) — Deep reference covering:

   • Single-level vs multi-level examples
   • Full config reference table with constraints
   • Complete MLM blueprint example (5-level referral with tiered achievements)
   • Attenuation design explanation
   • Performance notes
   • Composition with has_invite_achievements

3. references/invite-virality.md — Added "Multi-Level Referral Chains" section linking to the new reference file

4. Repackaged constructive-sdk-events.zip

Review & Testing Checklist for Human

• Verify the blueprint JSON examples are valid and match the actual parameter_schema
• Check that the attenuation explanation is clear for non-technical readers

Notes

Low-risk docs-only change. No code modifications.

Link to Devin session: https://app.devin.ai/sessions/2ad5d38ab24e4d43ba283eb9ae8b5c81
Requested by: @pyramation

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