feat(community): add sponsorship launch kit

[Codewriter90x]

Summary

• adds a dedicated Why sponsor HitTheKit? section to the README
• aligns FUNDING.md with source-only publishing and the updated tier
• documents three one-time sponsorship tiers
• adds ready-to-use LinkedIn and community launch copy
• adds an original square social asset with fail-closed provenance tracking

Validation

• asset provenance contracts: pass (63 assets)
• public readiness contracts: pass
• git diff --check: pass

Content boundary

• source-only claims retained
• no public Unity binary promised
• no commercial music, charts, artwork, or third-party branding introduced