feat: implement issues #735, #736, #737, #738

[judikay95]

#735 – Add get_escrow_balance(invoice_id) view to invoice-escrow contract

• Read-only fn returns funded_amount for the invoice, or 0 if unknown
• Two unit tests: existing invoice returns correct balance, unknown returns 0

#736 – Add EscrowReleased event to invoice-escrow release function

• EscrowReleased { invoice_id, recipient, amount } struct added to types.rs
• Emitted via env.events().publish() after transfer succeeds in release()
• Unit test asserts event appears in emitted events list after release

#737 – Add /// doc comments to all public functions in factory contract

• initialize, deploy_invoice_contract, get_deployments, is_salt_used, and contract_deployed helper all have full Rust doc comments
• Documents params, return values, panics — no logic changes

#738 – Add storage snapshot tests for invoice state persistence

• storage_roundtrip_write_then_read_equals_original
• storage_partial_update_does_not_overwrite_unrelated_keys
• storage_missing_key_returns_none_without_panic
• All use Soroban persistent storage directly

chore: clean up .gitignore — sectioned, deduplicated, tests/snapshots/ excluded

closes #735
closes #736
closes #737
closes #738

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