wallet: construct_from_address_net does not validate base58 addresses against the network

[fpelliccioni]

Problem

kth_wallet_payment_address_construct_from_address_net(address, net) takes a kth_network_t parameter, but only uses it for CashAddr disambiguation — base58 addresses pass through without any network validation.

A testnet base58 address (m... / n... with version byte 0x6f) is accepted even when net == mainnet. The function name and signature imply full network validation, but the implementation falls back to from_string(address, net) which calls decode_base58 first, and if that succeeds it returns immediately without consulting net.

Expected behavior

Either:

• Option A (strict): Validate that the base58 version byte matches the requested network (e.g., mainnet_p2kh = 0x00 / mainnet_p2sh = 0x05 for mainnet, testnet_p2kh = 0x6f / testnet_p2sh = 0xc4 for testnet). Reject mismatches.
• Option B (permissive + documented): Keep the current behavior but document explicitly that net only disambiguates CashAddr parsing — base58 addresses are always accepted regardless of network.

Affected code

• C++: payment_address::from_string(string, config::network) in src/domain/src/wallet/payment_address.cpp
• C-API: kth_wallet_payment_address_construct_from_address_net in src/c-api/src/wallet/payment_address.cpp

Context

Introduced alongside the CashAddr global removal in PR #240 / #241. The config::network parameter was added to replace the global set_cashaddr_prefix(). The base58 path was intentionally left untouched at the time, but the API name implies broader validation than it delivers.