test(e2e): extract p2p gossip scenario skeleton and add category README

[spalladino]

PR 8 of 9 of the e2e round-2 consolidation. Scope: end-to-end/src/p2p/** only. Dedup + docs; no timing/config value changes.

DATA_DIR centralization

• Every p2p file previously repeated DATA_DIR = fs.mkdtempSync(...) at module scope plus a per-node fs.rmSync cleanup loop in afterEach. That boilerplate now lives on P2PNetworkTest: setup() creates one root temp dir (this.dataDir), dataDirFor(label) hands out a nested per-role path (multi-node createNodes appends -<index>; single-node factories use it verbatim), and teardown() removes the whole tree in one recursive delete.
• Adopted in all files (the 6 test files + reqresp/utils.ts); deleted the per-file fs/os/path imports, mkdtempSync, and cleanup loops. This also fixes a small pre-existing leak: the old mkdtempSync root dir was never removed (node data went into sibling -N dirs), whereas nodes now live under the root that teardown deletes.

Gossip scenario skeleton (shared.ts)

• runGossipScenario(opts): Promise<AztecNodeService[]> — the shared bootstrap→createNodes→mesh→account→(submit)→verify flow. Varying parts are options/callbacks: numValidators, bootNodePort, txsPerNode (0 skips submission), submitSequentially, mesh (topic/peer-count overrides), checkpointSource ('first-tx' | 'first-published'), beforeCreateNodes, createExtraNodes, beforeSubmit, afterVerify. Returns the validator nodes for teardown tracking.
• Extracted the repeated building blocks it uses: verifyAttestationSigners(t, nodes, checkpoint) (recovers signers, asserts each ∈ validator set, returns signers), getPublishedCheckpointForTx(node, txHash), waitForFirstPublishedCheckpoint(t, nodes), waitForNodesToSync(t, nodes), and maybeCheckQosAlerts(logger) (the CHECK_ALERTS Grafana block that was copy-pasted in 4 files).

Merge: gossip_network_no_cheat.test.ts → gossip_network.test.ts

The two files shared the skeleton but genuinely differ in network bootstrap (cheat MultiAdder registration vs. on-chain addL1Validator CLI flow) and config, so they became two describes in one file, each with its own beforeEach — the mesh is not re-spun per it.

• describe('cheat-registered validators') — was gossip_network.test.ts. Effective node config unchanged: 4 validators, startProverNode:false, SHORTENED_BLOCK_TIME_CONFIG_NO_PRUNES + aztecSlotDuration:24, aztecEpochDuration:4, blockDurationMs:10000, slashingRoundSizeInEpochs:2, slashingQuorum:5, listenAddress:127.0.0.1, inboxLag:2; plus a p2p-only prover node and a re-execution monitoring node; proven-block assertion preserved.
• describe('on-chain-registered validators (no cheats)') — was gossip_network_no_cheat.test.ts. Effective node config unchanged: 4 validators, SHORTENED_BLOCK_TIME_CONFIG_NO_PRUNES + aztecSlotDuration:24, blockDurationMs:10000, minTxsPerBlock:0, listenAddress:127.0.0.1, inboxLag:2; full-mesh wait on the proposal/checkpoint topics; all on-chain registration assertions preserved. BOOT_NODE_UDP_PORT default stays 4500 (now shared with the cheat describe, which honored the BOOT_NODE_UDP_PORT env override — the default is unchanged).

Both describes run sequentially in one CI container reusing port 4500 (teardown fully frees ports before the next beforeEach).

fee_asset_price_oracle_gossip.test.ts

Kept as its own file, now on the shared skeleton with txsPerNode: 0 (it submits no txs — empty blocks carry the price) and checkpointSource: 'first-published'. Effective node config unchanged: 4 validators, startProverNode:false, SHORTENED_BLOCK_TIME_CONFIG_NO_PRUNES + aztecSlotDuration:12, aztecEpochDuration:4, slashingRoundSizeInEpochs:2, slashingQuorum:5, listenAddress, inboxLag:2, plus a prover node (minTxsPerBlock:0). Both oracle-convergence rounds and final assertions preserved.

Other files

• preferred_gossip_network.test.ts adopted verifyAttestationSigners + maybeCheckQosAlerts (its signer-check block was identical, keyed off the validators array), plus the DATA_DIR change. Node config and the expect(signers.length).toEqual(validators.length) assertion unchanged.
• late_prover_tx_collection.test.ts, rediscovery.test.ts, reqresp/reqresp.test.ts + reqresp/utils.ts: DATA_DIR change only; node config unchanged.
• Added p2p/README.md (the missing category README) matching the multi-node/single-node structure: what belongs here (real-libp2p subjects — discovery, req/resp, mesh, peer auth, transport), what does not (consensus/slashing/sentinel → multi-node), the P2PNetworkTest entrypoint, the shared skeleton/helpers, and the post-merge file inventory.

Behavior notes

• No dropped assertions.
• Two benign, non-asserting implementation changes, documented for reviewers: (1) in the no-cheat describe setupAccount() now runs before the first-checkpoint wait instead of after — both still precede tx submission, and setupAccount() only registers the account in the PXE (no on-chain effect), so the "checkpoint published before submit" invariant is preserved; (2) the merged submit path uses waitForTxs(...) for both describes rather than the no-cheat file's per-tx Promise.all(waitForTx) — same asserted property (all txs mined).

Local runs (one file at a time; real-libp2p tests bind fixed ports)

All green:

File / describe	wall
gossip_network — cheat-registered	121s
gossip_network — on-chain (no cheats)	172s
fee_asset_price_oracle_gossip	120s
rediscovery	69s
late_prover_tx_collection	47s
reqresp/reqresp	121s
preferred_gossip_network	141s

CI selection is glob/regex-based (src/p2p/**/*.test.ts, src/p2p/*.test.ts, .test_patterns.yml's src/p2p/.*), so deleting the merged-away file needs no CI-config edits.