docs: remove Drips Wave references and reframe for open source contributors

[Oyinkans0la12]

Summary

SplitStream targets open source teams and contributors generally, not a specific rewards program. This removes every Drips Wave reference from the repo and rewords the affected copy accordingly.

Changes

• README.md — remove the 📋 Wave community bullet linking to Drips Stellar Wave; reword "Wave-cycle points payouts" → "Cycle-based points payouts"
• docs/introduction.md — "lets a Drips Wave team turn..." → "lets an open source team turn..."; "Wave payouts" → "Contributor payouts"
• docs/protocol-mechanics.md — "Wave-cycle points payouts" → "Cycle-based points payouts"
• contracts/vault/src/lib.rs — same doc-comment reword
• contracts/vault/src/types.rs — "one Wave cycle payout" → "one payout cycle" (doc comment only)
• assets/splitstream-banner.svg — aria-label "pooled payouts for contributor waves" → "pooled payouts for open source contributors"

Verification

• Case-insensitive search for drip / wave across the repo returns zero matches
• Rust changes are doc comments only — no behavior change; CI will confirm cargo check/test/clippy/build