fix(net): validate peer transactions before mempool insertion

[ace-degen]

Summary

The P2P NewTransaction handler in lib/node/net_task.rs inserted peer-supplied
transactions into the mempool via MemPool::put without first running
State::validate_transaction. The RPC submission path (Node::submit_transaction)
does validate. Because of this asymmetry, a transaction received from a peer with
an invalid signature (or one that fails balance/fee checks) is accepted into the
mempool and re-broadcast to all peers, even though it can never be included in a
valid block.

Any peer can exploit this to flood the network's mempools with invalid transactions:

• network-wide mempool poisoning / wasted bandwidth (each node accepts and relays),
• non-mining nodes never run the template builder, so the junk stays resident in
  their mempools indefinitely (unbounded growth from free, keyless traffic).

(The block-template path is self-healing: Node::get_transactions re-validates and
drops invalid txs before building a template, so they never reach a block — the
impact is relay amplification and mempool bloat, not poisoned templates.)

(Severity: network-level DoS — no direct theft, since invalid transactions are still
rejected at block-connect time.)

Root cause

The handler called only regenerate_proof (which generates a Utreexo proof, not a
validation step) followed by mempool.put (which only guards against double-spends
within the mempool). Neither verifies signatures, address binding, input existence,
or fees.

Fix

Run State::validate_transaction in the NewTransaction handler before
MemPool::put, mirroring the RPC path.

Test

Adds lib/mempool.rs::p2p_validation_bypass_tests, a runtime regression test that
builds a transaction spending a funded UTXO but signs it with the wrong key, then
asserts:

1. State::validate_transaction rejects it with an Authorization error, but
2. MemPool::put on its own accepts it and the invalid transaction is left resident
   in the mempool.

running 1 test
[validate_transaction(forged tx) => Err(Authorization)]
test mempool::p2p_validation_bypass_tests::p2p_path_accepts_transaction_that_validation_rejects ... ok
test result: ok. 1 passed; 0 failed