Derive default network ranges and switch overlapping networks

Every first network previously proposed 10.13.37.0/24. Independently created
networks therefore collided when a device joined more than one of them.
Use the existing stable NetworkId-derived /24 in 10/8 for every new network,
independent of activation order or peer availability. Legacy and explicit
signed ranges outrank the derived default; remaining range disagreements
still use author-id ordering. The derivation, identity labels, handshake and
signed record encoding stay unchanged. There is no creator, timestamp or vote.

Reserve ranges before starting network actors and serialise start, stop and
switching through the agent lifecycle lock. Refuse equal or nested overlaps
without changing a network's subnet locally. A late authenticated range that
collides suspends only its network before updating plugins or the interface.
Coalesced watch notifications let the agent clean up outside the network actor
and reconsider suspended networks when ranges change.

An explicit switch stops every blocker before activating the chosen network.
Suspended networks retain their startup request and resume whenever their
ranges become free, including switching from a wide range to a narrow one.
A manual stop cancels that request even for an already suspended network.
Persist preference order and enabling together in an SQLite transaction so
the user's selection survives restart; no database schema migration is needed.

CLI status and network listing show blocker names, ids, ranges and the start
command. Network start switches explicitly and reports suspended networks;
joining an overlapping network reports it as configured but inactive. The
tray adds conflict details, an amber icon, Switch to this network and Keep
stopped actions. Local IPC wire version is 19 for the extended reports, so
restart the agent and tray together. Release versions and peer wire formats
are unchanged. Update the user documentation and compatibility tests.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets with TSUNAGI_CONTROL_SOCKET
  removed from the test environment: 486 passed, 2 pre-existing ignored tests
- final focused range_conflicts integration suite after making discovery
  fixtures deterministic: 6 passed

Earlier validation attempts caught outdated diagnostic assertions, a new
test helper scope error, and the inherited system control socket override.
Those were corrected before the successful runs. Plain cargo was unavailable
in the host shell, so validation used the Nix development toolchain. macOS,
Windows, privileged host TUN operations and manual tray interaction were not
exercised.

README.md

@@ -81,7 +81,17 @@ tsng join -n home -s tsn1…        # every other machine
 tsng status                       # who is online, over which path
 ```
 
-Every member gets an address in `10.13.37.0/24` and a name like `<host>.home`.
+Every new network gets a stable pseudorandom `/24` in `10.0.0.0/8`, derived
+from its network id. Its members agree on that range even when starting offline,
+and each gets a name like `<host>.home`. Existing signed ranges are preserved.
+
+If two configured networks have overlapping ranges, the agent runs only one.
+`tsng status`, `tsng network` and the tray show which network blocks the
+other. `tsng network start <name-or-id>` or the tray’s “Switch to this network”
+button suspends the conflicting networks. They resume automatically when the
+selected network stops; the selection survives restarting the agent. Explicitly
+stopping a suspended network cancels its automatic resume.
+
 Machines are called `tsunagi-<id>` until you rename them: `tsng id hostname laptop`.
 
 ### Exit nodes