refactor: introduce SettlementError contracterror enum

[Vvictor-commits]

Closes #330

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Summary

Replaces raw panic strings in CalloraSettlement with a typed #[contracterror] enum so callers and indexers can handle errors deterministically, and removes embedded string literals to shrink WASM.

Changes

contracts/settlement/src/lib.rs

• Add SettlementError enum with 8 stable u32 codes (1–8)
• Replace all panic!() / unwrap_or_else(|| panic!()) error paths with env.panic_with_error(SettlementError::*)
• Add MAX_BATCH_SIZE = 50 constant (was missing — compile error)
• Fix storage key consistency: init and set_vault now write via StorageKey::Admin / StorageKey::Vault enum keys, matching what get_admin / get_vault read
• Fix batch_receive_payment to use StorageKey::DeveloperBalance persistent storage, consistent with receive_payment and get_developer_balance
• Remove unused string constants that would fail clippy -D warnings

contracts/settlement/src/test.rs + test_views.rs

• Replace catch_unwind / panic_message with try_* client methods and InvokeError::Contract(code) assertions
• Fix test_settlement_initialization: remove assertion on StorageKey::DeveloperIndex (written lazily, not at init)

docs/interfaces/settlement.json

• Add top-level errors table with all 8 codes
• Replace panics string arrays with typed errors arrays per function

Error codes

Code	Variant	When
1	NotInitialized	Called before init
2	AlreadyInitialized	init called twice
3	Unauthorized	Caller is not vault or admin
4	AmountNotPositive	amount <= 0
5	DeveloperRequired	to_pool=false with no developer
6	DeveloperMustBeNone	to_pool=true with a developer
7	PoolOverflow	i128 overflow on pool balance
8	DeveloperOverflow	i128 overflow on developer balance

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[greatest0fallt1me]

Merged via direct push to main (admin)