fix(sync): trust only initial validators + disable peer_private

[RomThpt]

Summary

Two follow-on fixes after #9 to make the cross-impl late-join sync actually run end-to-end.

1. Trusted UNL = initial validators only

The topology was being built with numInitial + 1 keys so the late joiner had a slot, but WithValidatorConfig uploads the full key set as the trusted UNL on every node. rippled then applies its BFT rule (quorum = ceil(0.8 * trusted_count)), which on a 3-key list demands quorum=3 — and the 2-node initial network never validates.

rxrpl honored the configured XRPL_VALIDATION_QUORUM directly, so the rxrpl-only test passed and masked this. Drop the late joiner from the trusted set; it's a pure observer that catches up via received validations and doesn't need to be trusted itself.

2. peer_private=0 for sync tests

xrplsim/topology.go:EnvForNode hardcodes XRPL_PEER_PRIVATE=1. With that, rippled servers respond to non-fixed inbound connections with a 503 redirect (processResponse: failed to connect to peer: redirected). The late joiner's IP is not in the initial nodes' [ips_fixed] at boot, so it can never establish a peer link.

Override to 0 in the sync suite — rxrpl ignores the flag entirely (which is why the rxrpl-only path was unaffected by either issue).

Result on `--client rxrpl,rippled` (4 sub-tests):

• ✅ `initial=rxrpl late=rxrpl` — PASS
• ❌ `initial=rxrpl late=rippled` — fail (rippled catch-up from rxrpl-only UNL — separate issue, observable in rippled logs as STATE->disconnected despite peer link up)
• ✅ `initial=rippled late=rxrpl` — PASS (the cross-impl signal: rxrpl follows a rippled-validated chain end-to-end, account balance verified)
• ❌ `initial=rippled late=rippled` — fail (test-side: payment isn't yet in the first 10 ledgers under rippled's pacing — needs a longer tx-confirmation wait)

Why the partial fail is still progress

Before this PR the suite reported 0/1 passing in mixed mode and never actually exercised cross-impl sync. After this PR the canonical mainnet-shaped scenario "a fresh rxrpl observes a rippled-validated network and catches up" passes empirically, account state included.

Test plan

• `go build ./simulators/sync/` clean
• `--client rxrpl` (single impl) still passes the single test
• `--client rxrpl,rippled` reports 4 sub-tests with 2 passing including `initial=rippled late=rxrpl`
• follow-up: investigate rippled→rxrpl-validated catch-up path (the failing test 2)
• follow-up: extend WaitForLedger or move the tx earlier so rippled→rippled sees the payment