feat(devnet): add loopback_only flag for LAN testnet support

[Nic-dorman]

Summary

Adds an opt-in loopback_only: bool field on DevnetConfig (default
true) so the devnet harness can bind nodes to all interfaces instead
of loopback-only. Default-true preserves bit-identical behavior for all
existing callers; loopback_only: false enables a cross-host LAN
devnet path.

This is purely a dev-orchestration change — Devnet/DevnetConfig
are dev-only and not on the production node runtime path.

Why — LAN testnets for app testing

Every developer running an Autonomi-network app today needs to spin up
their own 25-node + Anvil stack locally to test against. That doesn't
scale across mixed-OS test fleets (Windows + macOS + Linux laptops on
one team). The motivating use case here is a "lab box" that hosts the
devnet once, with other devices on the same LAN acting as thin testers
that fetch the bootstrap manifest and run ant file upload/download.

A sibling tool (separate repo, testnet-lan) serves the rewritten
manifest over HTTP. It's not in this PR — that lives independently and
consumes whatever this exposes.

Changes

src/devnet.rs:

• New loopback_only: bool field on DevnetConfig.
• Default impl sets loopback_only: true.
• start_node bifurcates the saorsa-core builder:
  • loopback_only=true: identical to today — .local(true) only.
  • loopback_only=false: .local(false).allow_loopback(true). The
    explicit allow_loopback is required because saorsa-core's
    local(false) would otherwise default it to false, which would
    reject the loopback bootstrap addresses devnet nodes still use to
    dial each other on the same lab host.

29 insertions, 3 deletions. No changes to manifest types or other public
API.

Backwards compatibility

Caller	Result
DevnetConfig::default()	loopback_only: true — historical behavior
DevnetConfig::minimal() / small()	inherits ..Self::default() — historical
Struct-update syntax (DevnetConfig { evm_network: …, ..default() })	inherits — historical
Exhaustive struct literals	would need to add the field; standard pattern is ..Default::default() so this is unlikely in practice

Compatibility with saorsa-labs/saorsa-core#105

This change is complementary, not in conflict, with
saorsa-labs/saorsa-core#105
(LAN-scoped DHT addresses). The two operate at different layers:

• This PR: decides which interfaces devnet nodes bind to.
• saorsa-core#105: decides which addresses get shared with which
  peers and tags them as AddressType::Lan.

When both land, the LAN testnet path improves: with loopback_only=false,
saorsa-transport's listen_addrs() reports LAN addresses, and
saorsa-core#105's peer_can_receive_lan_addresses heuristic correctly
hands LAN-tagged entries to clients on the same network. saorsa-core#105
alone wouldn't enable cross-host LAN devnets — without this PR, the
devnet has no LAN address to share. They depend on each other for
the full LAN-testnet story.

maybe_swap_same_lan_ip_to_loopback from #105 only triggers when a peer
announces our own LAN IP back at us (intra-host case), which doesn't
fire for clients on different hosts. No interaction issue.

Test plan

• cargo build --lib clean
• cargo test --lib: 448/448 passing on the unchanged default path
• cargo fmt --all -- --check clean
• No new clippy warnings (one pre-existing never_loop lint at
  line ~770 fires on bare main too; not introduced here)
• Cross-host LAN smoke test against a second machine — pending the
  sibling testnet-lan tool reaching public review

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds an opt-in loopback_only switch to devnet orchestration so devnet nodes can be bound to all interfaces for cross-host LAN scenarios while keeping the default behavior unchanged for existing callers.

Changes:

• Added loopback_only: bool to DevnetConfig with a default of true (preserves historical loopback-only behavior).
• Updated Devnet::start_node to choose between local(true) and local(false); when local(false) is used, allow_loopback(true) is set to preserve intra-host loopback bootstrap behavior.

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[Copilot]

The docs for loopback_only read a bit like setting it to false is sufficient for cross-host clients. In this module bootstrap_addrs() (and the ant-devnet manifest it produces) still always uses Ipv4Addr::LOCALHOST, so remote clients will need a rewritten/overridden bootstrap list even when nodes bind to 0.0.0.0/::. Consider adding an explicit note to avoid confusion about what this flag does/doesn’t change.

Suggested change

	///
	///
	/// Note: this only affects the addresses nodes bind/listen on. It does
	/// not rewrite bootstrap addresses produced by this module (including
	/// [`Self::bootstrap_addrs()`] and the generated `ant-devnet` manifest),
	/// which still use localhost addresses. Remote clients on other hosts
	/// must therefore override or rewrite the bootstrap list to use a
	/// reachable LAN/WAN IP for the devnet host.
	///