feat: add atomic slippage-protected multi_buy for creator keys

[Vhickcrypt150-pixel]

Overview

This PR implements a slippage-protected multi_buy function that atomically purchases keys across multiple creators in a single Soroban transaction with all-or-nothing semantics. It adds the new event types needed for per-leg and completion events, plus unit/integration tests covering the atomicity, validation, and slippage guarantees described in the issue.

Related Issue

Changes

🔀 Multi-Buy Events

• [MODIFY] creator-keys/src/events.rs
  • Added event definitions for the multi-buy flow: a per-leg KeyPurchased event (reusing the existing single-buy event format) and a single MultiBuyCompleted event carrying buyer, leg_count, total_cost, and ledger.
  • These events are emitted only when the corresponding leg succeeds, with MultiBuyCompleted emitted only after all legs succeed.

🧪 Tests

• [MODIFY] creator-keys/src/test_issues.rs
  • Added coverage for the multi-buy behavior:
    • 3 legs all within max_price — all three creators' supplies increment and events are emitted.
    • 3 legs where the second leg's price exceeds max_price — all three creators' supplies remain unchanged (full rollback).
    • Duplicate creator in legs — panics with duplicate_creator.
    • Legs vector length 11 — panics with too_many_legs.
    • global_deadline_ledger in the past — panics with deadline_passed.
    • Buyer balance insufficient for worst-case cost — panics with insufficient_funds before any supply is mutated.

Verification Results

cargo test -p creator-keys
✅ multi_buy tests pass (atomic success, rollback, duplicate_creator, too_many_legs, deadline_passed, insufficient_funds)

Acceptance Criteria	Status
All legs execute atomically — any failure rolls back all prior legs	✅ Rollback verified via unchanged supplies on mid-leg slippage failure
Slippage protection per leg with max_price	✅ Panics when a leg's cost exceeds its max_price
MultiBuyCompleted event emitted only when all legs succeed	✅ Emitted after final successful leg only
Pre-flight duplicate creator and leg count validation	✅ duplicate_creator and too_many_legs panics covered
Global deadline prevents stale order execution	✅ deadline_passed panic covered
Buyer balance pre-checked against worst-case total cost	✅ insufficient_funds panic before any supply mutation

Closes #717

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