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/protocol.md

@@ -9,7 +9,7 @@ Two versions exist and are independent:
 
 - **Identity scheme**, `tsunagi-network-id-v1`. Frozen. Changing it creates a
   different network space for the same name and secret.
-- **Control protocol**, ALPN `tsunagi/ctrl/2`, `PROTOCOL_VERSION = 2`.
+- **Control protocol**, ALPN `tsunagi/ctrl/3`, `PROTOCOL_VERSION = 3`.
 
 Upgrading the crate or bumping the control protocol must never change an
 existing `NetworkId`.
@@ -191,7 +191,7 @@ transport connection, not supplied by the frame. Intermediate nodes cannot
 decrypt or authenticate the inner WireGuard payload; the destination does that.
 Flow ids are routing hints, not authorization proofs.
 
-Control ALPN 2 carries `Reachable { links: [{ peer, protocol }] }`. Each row
+Control ALPN 3 carries `Reachable { links: [{ peer, protocol }] }`. Each row
 belongs to the authenticated sender and is replaced atomically, expires after
 90 seconds, and is withdrawn on session closure. Only compatible authenticated
 members enter a protocol's graph; local edges always come from actual links.
@@ -215,7 +215,7 @@ not affect other networks.
 
 | message | meaning |
 |---|---|
-| `Announce { hostname, capabilities }` | this agent's hostname and IP-plugin capabilities |
+| `Announce { hostname, capabilities, broadcast }` | this agent's hostname, IP-plugin capabilities and local broadcast participation |
 | `Ping { seq, payload }` | small request used to verify the exchange |
 | `Pong { seq, payload }` | the echoed reply |
 | `State { records }` | a snapshot of signed records, merged into what the receiver holds |
@@ -223,6 +223,11 @@ not affect other networks.
 | `Reachable { links }` | sender's current direct data links, scoped by protocol |
 | `Bye { reason }` | graceful goodbye; not a revocation of anything |
 
+`broadcast` is a per-network local opt-in, enabled by default and treated as
+false until an authenticated announcement arrives. It governs IP broadcast
+fanout and local admission; encrypted transit still uses the existing envelope.
+See [broadcast.md](broadcast.md) for scope and persistence.
+
 A `State` snapshot is merged, never substituted: an author missing from it is
 left untouched. Each record carries its own signature, so a peer forwarding
 somebody else's record cannot alter it, and a record that fails verification