Added per-network LAN broadcast relay

LAN game discovery previously dropped IPv4 broadcasts at TUN ingress. Carry
limited and subnet-directed UDP broadcasts to authenticated, opted-in members
of the source network, including destinations reached through mesh relays.
Preserve the original IP/UDP bytes and deliver received broadcasts only to the
local TUN; never reflood them or expose another pair's plaintext at transit.

Build immutable recipient snapshots on address and participation changes. The
origin sends one ordinary end-to-end encrypted copy per recipient; the existing
fast, bounded-hop transport router remains unchanged. Validate UDP framing,
source ownership and destination admission without game-specific port rules.
Keep network domains isolated and refuse implicit gateways to physical LANs.
The separate broadcast policy/domain layer is the extension point for future
authorized subnet exports; physical capture, bridging and LAN deduplication
are deliberately not implemented yet.

Persist default-on participation independently for each local network. Add
join --no-broadcast/--broadcast and network broadcast <id> [on|off], including
live updates and authenticated announcements. Joining without a flag preserves
the saved choice. Opt-out stops local origination and delivery, while opaque
unicast transit for other members keeps working.

Migrate SQLite schema 3 to 4 without replacing identities or signed state.
Use control ALPN 3 and local IPC protocol 14 for the new announcement/request
shapes; update peers and restart running agents together. The data ALPN 4
envelope remains unchanged. No release version bump, tag or push is included.

Document agent-owned commits in AGENTS.md: short English subjects, explanatory
bodies, scoped staging, honest validation, and repository-local fallback author
AB <ab@hexor.cy> only when an effective name/email is missing. Release actions
remain the user's responsibility.

Validation on Windows: cargo fmt --all -- --check; cargo check --locked
--workspace --all-targets; cargo clippy --locked --workspace --all-targets --
-D warnings; release workspace/all-target tests: 313 passed. The two existing
SQLite wipe failures (a_wipe_removes_everything_and_the_next_start_is_a_stranger
and wiping_twice_is_as_ordinary_as_wiping_once) were explicitly skipped; the
public-DHT smoke test and forwarding benchmark remain ignored by default.
New coverage exercises real iroh/WireGuard multihop fanout, single delivery,
runtime opt-out, unicast replies, domain isolation, malformed input and schema
migration. TUNs are in-memory; actual games and OS adapter selection were not
tested.

docs/broadcast.md

@@ -0,0 +1,91 @@
+# LAN discovery broadcasts
+
+Broadcast participation is a **local setting of each network**, enabled by
+default. Different networks on the same agent can have different settings;
+each member advertises whether it accepts broadcasts over its authenticated
+control session. It is not a network-wide vote or signed shared configuration.
+
+```sh
+tsunagi join -n games --no-broadcast
+tsunagi join -n games --broadcast
+tsunagi network broadcast <network-id-or-prefix> off
+tsunagi network broadcast <network-id-or-prefix> on
+tsunagi network broadcast <network-id-or-prefix>
+```
+
+Joining without either flag preserves the saved choice. The `network broadcast`
+command can change a running network immediately or configure it while the
+agent is stopped. `status` reports the local setting. There is no global `up`
+override: starting the agent preserves every network's independent policy.
+
+## Delivery
+
+At local TUN ingress, the IP router recognizes IPv4 UDP addressed to the limited
+broadcast `255.255.255.255` or the configured overlay subnet's directed broadcast
+(for example `10.13.37.255` for `/24`). These address forms follow
+[RFC 1122 section 3.3.6](https://www.rfc-editor.org/rfc/rfc1122#section-3.3.6).
+There is no port allowlist: discovery requests and server announcements on
+game-specific UDP ports use the same mechanism. IPv4/UDP lengths are checked
+before fanout, including handling IP fragments. IPv6 multicast, mDNS, DHCP
+bridging, Ethernet/IPX and physical LAN capture are outside this feature.
+
+The local source address selects exactly one broadcast domain. A limited
+broadcast is never copied into all networks merely because they share a TUN.
+A directed broadcast must belong to that source's network. Unspecified sources
+and physical LAN sources have no authorized domain yet. Recipients are live,
+authenticated, opted-in members with signed overlay address ownership. The
+immutable recipient list is rebuilt with address/policy changes, not per packet.
+
+The origin creates **one encrypted unicast copy per eligible peer** and hands it
+to the existing shortest-path router. Each can travel through several transport
+links. Transit handles opaque ciphertext with the existing hop limit and does
+not create another fanout. The original IP source, destination, TTL, UDP ports,
+checksum and payload remain unchanged. Reply packets use ordinary unicast.
+There is no broadcast retransmission timer or application-payload deduplication:
+games may legitimately repeat the same discovery query. WireGuard replay checks
+reject duplicated encrypted packets delivered by the transport.
+
+At the destination, the usual authenticated-source ownership check still runs.
+Broadcast admission also checks the receiving network's setting, the sender's
+announced participation, the destination domain and UDP framing. Disabling it
+takes effect locally even while a remote sender still has an older announcement.
+Received broadcasts terminate in the local TUN and never enter the fanout path.
+An ordinary decrypted packet must target this host's address in that network;
+the TUN is not a transit gateway to another overlay or physical subnet.
+
+Opt-out disables originating and accepting broadcasts on that host/network.
+It does not disable opaque unicast transit for other members: intermediate
+routers cannot inspect another pair's encrypted IP payload.
+
+## Host behavior and verification
+
+The game must send discovery through the Tsunagi interface. Traffic bound to a
+physical adapter never reaches this TUN, and this feature does not capture it.
+The suite tests real agents, authenticated iroh links and WireGuard encryption
+with memory TUNs: limited/directed UDP discovery, multihop fanout, single-copy
+delivery, opt-out, unicast replies, malformed input, source validation and
+network isolation. It does not launch CS 1.6 or Warcraft III or establish that
+every game/host chooses the virtual adapter automatically.
+
+## Future exported LANs
+
+`overlay::broadcast` owns domain selection and recipient policy separately from
+IP parsing, encryption and encrypted transit. A future LAN adapter should feed
+an explicitly authorized ingress domain into this layer, and export routes
+through the corresponding source/destination admission policy. It must not
+relax address checks globally or reinterpret all `.255` addresses as broadcasts.
+Multiple gateways to the same LAN will need origin identifiers and bounded
+duplicate suppression at LAN ingress/egress before physical rebroadcast is
+enabled. No subnet export or host forwarding configuration is added here.
+
+## Compatibility
+
+Control ALPN is `tsunagi/ctrl/3` because `Announce` now carries participation.
+The encrypted transit envelope and data ALPN `tsunagi/data/4` stay unchanged.
+Local control protocol 14 rejects commands from mismatched running binaries
+with a restart message instead of decoding a different request shape.
+Upgrade all members and restart the agent before using the new CLI.
+
+SQLite schema 4 adds `networks.broadcast` with default `1`. Existing network
+names, secrets, identity keys and signed records are retained. The old binary
+does not understand schema 4; the migration is not a downgrade mechanism.