feat(p2p): implement --disable-relay-tx flag

[RossKU]

Summary

Closes #228.

Implements the --disable-relay-tx CLI flag, allowing API-only nodes to opt out of transaction relay, reducing mempool overhead and bandwidth.

What it does

• Adds --disable-relay-tx CLI flag (also configurable via KASPAD_DISABLE_RELAY_TX env var or config file)
• Sets disableRelayTx in the version handshake message so peers know not to send us transactions
• Filters outbound tx broadcasts to skip peers that have set disable_relay_tx in their version handshake
• Drains incoming tx inv messages without acting on them when our node has relay disabled

Changes by file

• kaspad/src/args.rs — CLI arg, Args struct field, parsing, apply_to_config
• consensus/core/src/config/mod.rs — Config struct field + default
• protocol/flows/src/flow_context.rs — Populate disable_relay_tx in version message (resolves the TODO at line 716)
• protocol/flows/src/flowcontext/transactions.rs — Use broadcast_filtered / broadcast_to_some_peers_filtered to skip disable_relay_tx peers
• protocol/p2p/src/core/hub.rs — Add broadcast_filtered() and broadcast_to_some_peers_filtered() methods
• protocol/flows/src/v7/txrelay/flow.rs — Drain inbound tx inv when relay is disabled (v8 reuses v7 txrelay, so both versions are covered)

Design decisions

• Both directions: We don't broadcast tx inv to peers who set the flag, AND we don't request txs if we set the flag ourselves
• Drain, don't unregister: The relay flow is still registered (to avoid breaking protocol expectations) but drains inv messages without requesting transactions
• Mining safe: Mining uses RPC + mempool, not P2P relay. Nodes with --disable-relay-tx can still accept transactions via RPC

Out of scope: starvation mitigation

As noted by @coderofstuff in #228, if a node wants txs but all its peers have disable_relay_tx=true, no transactions arrive. Mitigation (peer eviction + reconnection logic) is deferred to a follow-up issue — in practice, disable_relay_tx nodes will be a small minority (API-only), so this scenario is unlikely.

Test plan

• cargo check --workspace passes
• cargo test --workspace passes (all tests)
• Manual: start kaspad with --disable-relay-tx, verify no tx inv messages are processed
• Manual: connect two nodes, one with flag set, verify txs are not relayed to it

🤖 Generated with Claude Code

[coderofstuff]

wdyt about the possible starvation issue @someone235 @michaelsutton?