feat: implement #771, #767, #766, #593

[NnamdiCyber]

Summary

Implements four vault core enhancements across the Soroban contract.

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Closes TTL-Legacy#771 — get_check_in_history_page with Cursor Pagination

New function: get_check_in_history_page(env, vault_id, cursor: u64, limit: u32) -> Vec<CheckInHistoryEntry>

• Cursor-based pagination — cursor = 0 returns the most recent entry, cursor = 1 the second-most-recent, etc.
• Limit is capped at 50 entries
• Returns entries newest-first
• Tests: pagination over 5 entries, boundary conditions, limit cap, and empty history

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Closes#767 — Maximum Number of Vaults Per Owner

• Added MaxVaultsPerOwner to DataKey (default: 50)
• Added VaultLimitReached error (code #82)
• Admin functions: set_max_vaults_per_owner / get_max_vaults_per_owner
• create_vault now rejects when owner already holds max_vaults_per_owner vaults
• Tests: default value (50), limit enforcement (third vault rejected), limit update (admin raises ceiling)

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Closes TTL-Legacy#766 — get_vault_age Function

New function: get_vault_age(env, vault_id) -> u64

• Returns seconds elapsed since vault.created_at
• Returns 0 if the vault does not exist (no panic)
• Tests: newly-created vault (age = 0), 1-hour-old vault, nonexistent vault

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Closes TTL-Legacy#593 — Token Fee Optimization

• Added TokenFeeConfig struct: accumulation_enabled, accumulation_threshold
• Added PendingWithdrawal struct for deferred transfers
• Modified do_withdraw to batch below-threshold amounts into pending list instead of immediate transfer
• Admin functions: set_token_fee_config, execute_pending_withdrawals
• Events: FEE_CONFIG_UPDATED_TOPIC, FEE_BATCH_EXECUTED_TOPIC
• Tests: default config, config get/set

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Files Changed

File	Changes
contracts/ttl_vault/src/types.rs	+3 DataKey variants, +2 structs, +2 event symbols
contracts/ttl_vault/src/lib.rs	~180 lines: 4 new public functions, modified create_vault & do_withdraw
contracts/ttl_vault/src/test.rs	~178 lines: 10 new test functions across all 4 issues