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/testing.md

@@ -76,6 +76,15 @@ links. Real encrypted 1280-byte TCP packets travel in both directions while the
 middle TUNs remain empty. A direct A—D link is enabled, then removed; the route
 switches back to the chain without replacing end-to-end tunnels.
 
+Broadcast tests use limited and directed UDP game discovery packets over real
+WireGuard tunnels, including a missing direct link. Each willing peer gets one
+copy; disabling reception/origination works at runtime and unicast still works.
+IP-router tests cover source/destination domain checks, malformed UDP and the
+absence of reflection into outgoing fanout. SQLite migration tests preserve v3
+identity/settings and check opt-out after reopen/rejoin. CLI tests cover default
+on, per-network opt-out, runtime updates and conflicting flags. Actual games and
+host adapter selection are not simulated by these tests.
+
 The ignored `forwarding_benchmark` measures the synchronous transit routine in
 release mode, excluding crypto and socket I/O. Run it explicitly as described
 in [routing.md](routing.md); it has no timing threshold in the default suite.