…ap.p2p-manifest

Fold in the fix from #20615 — the chain.toml v2 publish/seed stack runs
unconditionally on main and collides with the downloaded chain.toml.torrent
during post-OtterSync AddTorrentsFromDisk, aborting fresh mainnet sync with
"snapshot exists with a different name: chain.toml".

Gate the two call sites we still have after the downloader extraction:

1. node/eth/backend.go: wrap the ENR updater + NodeSource + delayed
   PublishLocalChainToml + StartChainTomlDiscovery + StartTorrentPeerManager
   block in --snap.p2p-manifest. Default syncs skip the whole v2 stack.

2. db/downloader/downloader.go:810: gate the post-download republish using
   d.manifestReady != nil (the internal signal that EnableP2PManifest was
   invoked by the backend). Covers the loop-internal publish in
   chainTomlDiscoveryLoop too.

The third call site from #20615 (initDownloader's initial publish at
backend.go:1461 on main) doesn't exist in our branch — initDownloader moved
to node/components/downloader/Provider.initDownloader as part of the
extraction, and that function doesn't call PublishLocalChainToml.

Default --snap.p2p-manifest=false ⇒ zero v2 behaviour ⇒ collision avoided.
Opt-in --snap.p2p-manifest=true ⇒ full v2 publish + discover + peer-manager
flow preserved.

Ref: #20615

…ap.p2p-manifest (#20615)

## Summary

- Fix fresh-mainnet-sync regression introduced with the chain.toml v2
merge: the publish/seed stack was running unconditionally and collided
with the downloaded `chain.toml.torrent` during post-OtterSync
`AddTorrentsFromDisk`, aborting the execution service with `snapshot
exists with a different name: "chain.toml"`.
- Gate the three unconditional v2 call sites on the existing
`--snap.p2p-manifest` flag (default `false`) so default syncs stay on
the pre-v2 path while opt-in users get the full v2 flow.

## Reproduction (before this change)

```
USE_STATE_CACHE=false ./build/bin/erigon \
  --datadir=<fresh> --chain=mainnet --prune.mode=minimal
```

Fails during stage `[1/6 OtterSync]` immediately after `Downloader
completed remaining snapshots`:

```
[EROR] Could not start execution service
  err="[1/6 OtterSync] after snapshot download: adding torrents from disk:
       adding torrent for chain.toml.torrent:
       snapshot exists with a different name: \"chain.toml\""
```

Collision originates in `getExistingSnapshotTorrent`
([db/downloader/downloader.go:1683](https://github.com/erigontech/erigon/blob/main/db/downloader/downloader.go#L1683))
— the locally-seeded `chain.toml` torrent is already registered under
that info-hash when the disk-scanned `chain.toml.torrent` tries to add.

## Change

Three gated sites, all tied to the pre-existing `--snap.p2p-manifest`
flag (usage: *"Discover snapshot manifest (chain.toml) from P2P peers
via ENR instead of using centralized preverified.toml"* — default
`false`):

1.
[node/eth/backend.go:541](https://github.com/erigontech/erigon/blob/main/node/eth/backend.go#L541)
— wrap the whole ENR updater / `SetNodeSourceFn` / delayed
`PublishLocalChainToml` / `StartChainTomlDiscovery` /
`StartTorrentPeerManager` block in `&&
backend.config.Snapshot.P2PManifest`.
2.
[node/eth/backend.go:1461](https://github.com/erigontech/erigon/blob/main/node/eth/backend.go#L1461)
— gate the initial publish in `initDownloader()`.
3.
[db/downloader/downloader.go:810](https://github.com/erigontech/erigon/blob/main/db/downloader/downloader.go#L810)
— gate the post-download republish using `d.manifestReady != nil`, which
is the internal signal that `EnableP2PManifest()` was invoked by the
backend. This also covers the loop-internal publish in
`chainTomlDiscoveryLoop` (only runs under P2P manifest mode).

Default `--snap.p2p-manifest=false` ⇒ **zero v2 behaviour** ⇒ collision
avoided.
Opt-in `--snap.p2p-manifest=true` ⇒ full v2 publish + discover +
peer-manager flow preserved.

## Test plan

- [x] `make lint` — 0 issues
- [x] `make erigon` — builds clean
- [x] `go test -short ./db/downloader/...` — passes
(`chaintoml_test.go`, `chaintoml_v2_test.go` call `PublishChainToml`
directly, unaffected)
- [x] `go vet ./node/eth/... ./db/downloader/...` — clean
- [ ] Manual: fresh `--chain=mainnet` sync with default flags completes
OtterSync and enters Headers stage without the collision error (reviewer
to confirm on their setup — original reproducer took ~13 min to hit the
crash point)
- [ ] Manual: fresh sync with `--snap.p2p-manifest=true` still performs
chain.toml discovery / publishing as before (v2 behaviour preserved)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>