feat: content curation vote, reputation decay, validator rewards, NFT fractionalization

[Joeloo1]

Summary

Implements four new features for the Stellar DeFi Vault:

• Content Curation Voting — Stake-weighted voting on off-chain content items (identified by hash). Stakers submit, vote (weight = staked amount), and admin closes voting. Max 100 open items, one vote per address per item.

• Reputation Decay — Time-based decay mechanism for reputation scores. Scores gradually decrease when stakers are inactive. Admin-configurable decay rate (bps per epoch) and epoch length. Decay applied lazily via get_reputation_score.

• Validator Rewards — Integration layer for Stellar validator nodes. Validator rewards are deposited separately, distributed proportionally to stakers by position size, and claimed via a dedicated path. Separate from base staking rewards.

• NFT Fractionalization — Splits NFT receipt into N fungible fraction tokens (2–1000). Position is locked while fractionalized — unstake blocked until all fractions returned and NFT reconstructed.

Acceptance Criteria

Content Curation (Issue #295)

• ContentItem struct with all required fields
• submit_content(user, content_hash) — max 100 open items
• vote_on_content(user, content_hash, approve) — weight = position.amount
• One vote per address per item — overwrite not allowed
• close_content_vote(admin, content_hash) — admin closes voting
• get_content_item(hash) / get_all_content_items() read-only queries
• content_approved event when votes_for > votes_against on close
• Tests: vote weight, double vote rejected, close outcome, max items

Reputation Decay (Issue #296)

• set_reputation_decay_rate(admin, decay_bps_per_epoch, epoch_ledgers)
• get_reputation_decay_config() read-only query
• apply_reputation_decay(user) public function
• get_reputation_score applies decay lazily
• Score never decays below 0
• reputation_decayed event
• Tests: inactive decays, active unchanged, floors at 0, rate 0 disables

Validator Rewards (Issue #297)

• set_validator_node(admin, node_address) / get_validator_node()
• ValidatorRewardPool balance tracked separately
• deposit_validator_rewards(validator_node, amount) — only node can call
• distribute_validator_rewards(admin) — proportional to position size
• ValidatorRewardBalance(Address) persistent per-user
• claim_validator_rewards(user) -> i128
• validator_rewards_distributed event
• Tests: deposits, proportional distribution, non-validator rejected, claims

NFT Fractionalization (Issue #303)

• FractionalizedNFT struct with owner and total_fractions
• fractionalize_nft(user, num_fractions) — min 2, max 1000
• reconstruct_nft(user) — burns all fractions, restores NFT
• Position locked while fractionalized — unstake blocked
• FractionalRegistry persistent storage
• nft_fractionalized / nft_reconstructed events
• Tests: locks position, reconstruction unlocks, partial burn fails, non-owner rejected

Closes #295
Closes #296
Closes #297
Closes #303

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