Allocate IPv4 addresses and keep them, as signed state

Derived IPv4 addresses could not survive anything: they changed with the
range, and there was no way for a member to come back to the one it had.
Addresses are now allocated and recorded as signed facts, which is the
first slice of the model in docs/sync-model.md.

src/state/ holds one record per author per network, carrying that author's
complete current statement, signed with its persistent device key over a
length-prefixed canonical encoding. Merging follows the model's rules: a
higher version wins, an older one never rolls back a newer, duplicates are
idempotent, absence from a snapshot is not deletion, and a same-version
conflict is resolved identically on every replica and reported rather than
letting replicas diverge. Records are persisted in state.sqlite, with the
record and the author's version counter committed in one transaction
before anything is announced, and distributed as a State control message
that is merged into what the receiver already holds.

No vote, deliberately, despite the request. A majority is not a trust root
here — anyone with the secret can mint identities — and a quorum would
stall with one peer online and diverge across a partition. Signatures plus
a deterministic merge converge without either failure mode: two members
claiming one address at once are resolved by the lower endpoint id, and
the loser allocates again with a higher version.

The range moved from the plugin to the agent, defaults to 10.13.37.0/24,
and is now agreed rather than configured per member: a joining agent
adopts what the network already uses, so --ipv4-range only matters for
whoever starts it. The announcement went back to identity only (version 3)
since the range travels in signed records now.

A release tombstone exists and merges correctly, but nothing emits one
yet.

116 tests. The headline ones: an address survives restarting both agents,
three members get three distinct addresses, and a member started with a
different range adopts the one in use. Confirmed by hand with two CLI
agents restarted end to end.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/testing.md

@@ -50,6 +50,15 @@ keeping the WireGuard identity, shutdown removing every interface, a forged
 overlay claim being rejected, and the core carrying the payload without
 interpreting it.
 
+Unit tests in `src/state/` cover the signed record model directly: tampering
+with any field breaks verification, a newer version wins while an older one
+never rolls back, two authors claiming one address resolve the same way no
+matter the merge order, one key used in two places is reported rather than
+silently merged, a release survives a late-arriving old claim, a bad record in
+a batch does not stop the rest, and allocation is deterministic, spread out,
+walks past everything taken and reports a full range instead of handing out a
+duplicate.
+
 `tests/local_control.rs` covers the local control socket end to end: a client
 asking a running agent for status over a real Unix socket, a leftover socket
 file being replaced while a live one is not, and the derived socket path