feat(sdk): add futurenet and standalone network config

[arkhindi12]

Closes #981.

What this adds

NETWORK_CONFIG — one entry per network the SDK can target, with the RPC URL,
the passphrase the SDK signs with, the explorer bases and the published
contract IDs.

import { NETWORK_CONFIG, contractIdFor } from './constants.js';

const { rpcUrl, networkPassphrase } = NETWORK_CONFIG.standalone;

network	RPC	passphrase
mainnet	soroban-mainnet.stellar.org	Networks.PUBLIC
testnet	soroban-testnet.stellar.org	Networks.TESTNET
futurenet	soroban-futurenet.stellar.org	Networks.FUTURENET
standalone	localhost:8000/soroban/rpc	Networks.STANDALONE

Explorer entries are EXPLORER_URLS itself, not copies of the strings, and
standalone is null because no public explorer follows a network that only
exists on your machine.

About the contract IDs — please read

The issue asks for predefined contract IDs on futurenet/standalone. I did not
add any, deliberately: StreamFi contract IDs are derived per deployment from
sha256(networkPassphrase + ContractIdPreimageFromAddress(deployer, salt)),
so a hardcoded ID would point at somebody else's contract — worse than none.

Instead contracts is null per role and contractIdFor() throws with the
next step spelled out:

No canonical factory contract ID is published for the 'futurenet' network —
StreamFi contract IDs are generated per deployment. Deploy the contracts and
pass the factory address explicitly, e.g. new ConduitClient({ network:
'futurenet', factoryAddress: '<your factory contract ID>' }).

If you would rather have a mutable registry that deployments write into, say
so and I will add it — but I did not want to ship an address that looks
authoritative and is not.

Tests

src/tests/network-config.test.ts — 7 cases: all four networks present, the
passphrases match what the SDK signs with, futurenet/standalone point at their
own RPC, explorer entries equal EXPLORER_URLS, no invented contract IDs, the
error message is actionable, and a published ID is returned when one exists.

Test Files  1 passed (1)
     Tests  7 passed (7)

Rebased on main after #1004. npx tsc --noEmit and this PR's test suite are green on the current head.

[arkhindi12]

Closing this — the issue was resolved by #1015, so this PR is superseded. Thanks for the review queue space.