Closes issue #77

PR: Dynamic NFT Evolution Contract (Issue #77)

Description

Implements a complete dynamic NFT evolution system where NFTs change properties based on player achievements, time, and milestones. This contract enables engaging game mechanics where player progression directly affects their in-game assets.

Key Features

• Milestone-Triggered Upgrades: Admin/verifier can manually evolve NFTs (level + rarity increases)
• Time-Based Evolution: Player-triggered automatic evolution after elapsed time
• Rarity Evolution: NFTs gain rarity through upgrades and fusion mechanics
• Fusion System: Combine two NFTs into a higher-rarity token (originals burned)
• Downgrade Support: Admin/verifier can reduce levels (penalties/corrections)
• Evolution History: Immutable log of all evolution events
• Access Control: Admin, verifier, and owner-based authorization

Acceptance Criteria (All Met ✅)

• NFT properties change based on conditions (level/rarity via evolution calls)
• Milestones trigger upgrades (admin/verifier evolve_milestone function)
• Time-based evolution works (player-triggered evolve_time with elapsed time check)
• Evolution history preserved (immutable Vec history tracking)
• Fusion creates new NFT (combines two → new higher-rarity token, originals burned)
• Contract compiles and tests pass (8 comprehensive unit tests)

Changes

New Files:

• contracts/dynamic_nft/Cargo.toml – Package manifest
• contracts/dynamic_nft/src/lib.rs – Full contract implementation (220+ lines)
• contracts/dynamic_nft/src/test.rs – Unit tests (8 tests covering all functionality)
• contracts/dynamic_nft/README.md – Complete documentation

Modified Files:

• Cargo.toml – Added contracts/dynamic_nft to workspace members

Contract Overview

Core Data Structure

pub struct DynamicNft {
    pub owner: Address,           // NFT owner
    pub level: u32,               // Evolution level (starts at 1)
    pub rarity: u32,              // Rarity rank (starts at 1)
    pub traits: String,           // Visual traits (updatable)
    pub metadata: String,         // Metadata URI
    pub history: Vec<String>,     // Evolution event log
    pub minted_at: u64,           // Timestamp for time-based evolution
}

Main Functions
initialize(admin) – Initialize contract
mint(minter, owner, metadata, traits) → token_id – Create new NFT
evolve_milestone(admin/verifier, token_id, level_inc, rarity_inc, traits) – Manual evolution
evolve_time(player, token_id, required_secs) – Time-based evolution
downgrade(admin/verifier, token_id, level_dec) – Reduce level
fuse(owner, token_a, token_b) → new_token_id – Combine NFTs
get_nft(token_id) → DynamicNft – Retrieve NFT data
get_history(token_id) → Vec<String> – Get evolution history
Access Control
Admin Only: initialize, add_verifier, remove_verifier
Admin/Verifier: evolve_milestone, downgrade
Owner Only: fuse
Open: mint, evolve_time, get_nft, get_history
Testing
All 8 unit tests pass:

✅ Mint and retrieval
✅ Time-based evolution with timer reset
✅ Milestone evolution (admin path)
✅ Downgrade by verifier
✅ Fusion with original token burn
✅ Authorization enforcement (3 panic tests)
✅ Verifier add/remove.

Run tests:
cargo test --manifest-path contracts/dynamic_nft/Cargo.toml

Build:
cargo build --manifest-path contracts/dynamic_nft/Cargo.toml --release