Reintroduce Smoldot light client support

Context

Light client support via Smoldot was removed because it was not working correctly. This issue tracks reintroducing it properly.

What was removed

• --light-client global CLI flag and dot chain add <name> --light-client command
• KNOWN_CHAIN_SPECS mapping with chain spec references for 12 chains
• createSmoldotProvider() function in src/core/client.ts
• lightClient?: boolean field from ChainConfig interface
• All related help text, completions, tests, and documentation

Previously supported chains

The KNOWN_CHAIN_SPECS mapping covered:

How createSmoldotProvider worked

async function createSmoldotProvider(chainName: string) {
  const { start } = await import("polkadot-api/smoldot");
  const { getSmProvider } = await import("polkadot-api/sm-provider");

  const entry = KNOWN_CHAIN_SPECS[chainName];
  const { chainSpec } = await import(entry.spec);
  const smoldot = start();

  if (entry.relay) {
    // Parachains need their relay chain initialized first
    const { chainSpec: relaySpec } = await import(relayEntry.spec);
    const relayChain = await smoldot.addChain({ chainSpec: relaySpec, disableJsonRpc: true });
    const chain = await smoldot.addChain({ chainSpec, potentialRelayChains: [relayChain] });
    return getSmProvider(chain);
  }

  const chain = await smoldot.addChain({ chainSpec });
  return getSmProvider(chain);
}

Key polkadot-api modules

• polkadot-api/smoldot — provides start() to initialize the Smoldot runtime
• polkadot-api/sm-provider — provides getSmProvider() to wrap a Smoldot chain into a JsonRpcProvider
• polkadot-api/chains/* — pre-built chain specifications for known chains

These are all sub-paths of the existing polkadot-api dependency — no additional packages needed.

Suggested approach for reintroduction

Rather than requiring an explicit --light-client flag, consider auto-detecting light client capability:

1. Maintain a KNOWN_CHAIN_SPECS mapping for chains that have embedded chain specs
2. When creating a client, if no --rpc override is provided and the chain is in KNOWN_CHAIN_SPECS, prefer the light client
3. If configured RPC endpoints exist, use those instead (light client as fallback or opt-in)
4. Add an explorerRpc field to KNOWN_CHAIN_SPECS for generating explorer links when using light client (since light client doesn't provide a representative RPC URL)
5. Ensure proper error handling and timeout behavior for light client connections
6. Update docs and tests accordingly

Start in src/core/client.ts and inspect the existing client creation flow, then review the removed CLI flag, chain-add command, help text, completions, tests, and documentation. Compare the proposed KNOWN_CHAIN_SPECS and Smoldot module usage with the current polkadot-api dependency. Done means light-client selection, RPC precedence or fallback, explorer links, errors, timeouts, tests, and documentation are defined and working for the listed chains.