Retire offline node claims with equal network-wide cleanup rights

Offline devices reserved names and IPv4 addresses indefinitely, leaving stale
members in the device history. Give every authenticated network participant
the same right to retire another device's observed signed claim without an
owner, quorum, permanent identity ban or secret change.

Add a separately signed retirement containing the target's original signed
claim as proof. Verify both signatures and network scopes, retain the strongest
proved cutoff per target with deterministic ties, and suppress claim versions
at or below it in membership, hostname ownership and IPv4 allocation. Keep the
old signed records and range agreement, and retain markers after a newer claim
returns so old replicas cannot resurrect obsolete reservations. A returning
device signs a newer claim, allocates against current holders and resets its
previous hostname priority rather than taking a reused name with an old time.

Commit local retirement and the requester's author counter atomically before
announcing it; receivers persist proofs before applying them. Bound history to
1024 targets by default, reject additional targets without evicting protection,
and treat corrupt or over-limit mandatory storage as an explicit error. Repeat
bounded marker batches with a per-session rotating cursor so queue pressure
cannot indefinitely starve later markers. Exclude retired claims and cached
hints from history-derived discovery without preventing a fresh authenticated
device from returning.

Expose Agent::forget_node through the shared local control API. CLI command
tsng network forget-node accepts exact network names, full ids or unique id
prefixes, and exact node hostnames, ids, prefixes or overlay IPv4 addresses.
Ambiguous selections change nothing. Recheck the observed version and refuse
self, online nodes, the selected exit node and absent or changed claims.
Add Forget with an identity confirmation to offline tray device rows. Widen
Show devices to 1440x640 with an 800x400 minimum to fit its contents.

Control ALPN/version advances to tsunagi/ctrl/7 and local IPC to 21; upgrade
all peers and restart the agent with matching CLI/tray clients. Data ALPN 5,
network identity derivation, handshake encoding and existing record signing
bytes remain unchanged. SQLite schema 9 adds retirement storage while retaining
identity, secrets, settings and signed claims. Document behavior and limits.

Validation on Linux through nix develop:
- cargo fmt --all -- --check
- cargo clippy --locked --workspace --all-targets -- -D warnings
- cargo test --locked --workspace --all-targets: 519 passed, 2 existing ignored
  tests; inherited TSUNAGI_CONTROL_SOCKET and TSUNAGI_CONTROL_GROUP were unset
- git diff --check and the built CLI forget-node help

Tests use real iroh authentication and SQLite. Cover restart, stale-replica
recovery, equal rights and network isolation, hostname reuse and return, twelve
marker batches through a two-message queue, malformed signatures and proofs,
invented future cutoffs, deterministic merge, capacity, migration and injected
counter-write rollback, selectors and cleanup through two real CLI processes.
Earlier iterations caught a private test API, a collapsible tray conditional
and a crash-based CLI test waiting for QUIC's idle timeout; corrected before
the final successful suite. Manual tray rendering, macOS/Windows, privileged
TUNs and public-network traversal were not run. The requested RC tag is
v0.1.0-rc.31; no push was requested.

docs/threat-model.md

@@ -38,6 +38,14 @@ Read this before relying on anything here. The protocol is in
   arbitrarily many identities, flood the network with records and collide with
   other participants' names. This is why a majority is not a root of trust.
   Signatures protect authorship; they do not make a participant honest.
+- **Cleanup rights are equal.** Any member can retire an observed signed claim
+  of another device, freeing its hostname and IPv4 reservation. A valid original
+  claim bounds the cutoff; a member cannot invent a future cutoff or forge the
+  target's release. The local API refuses online nodes, but a remote signature
+  cannot prove that a target was offline. A live target may publish a newer
+  claim; a malicious member can repeatedly disrupt reservations. Cleanup is
+  not expulsion or secret revocation. See
+  [sync-model.md](sync-model.md#offline-node-cleanup).
 - **Weak secrets.** This targets high-entropy secrets. There is no PAKE, so a
   short human passphrase can be guessed offline by anyone who can reach the
   handshake. Use `NetworkSecret::generate()`.
@@ -55,8 +63,10 @@ Read this before relying on anything here. The protocol is in
   having iroh under it does not make it so.
 - **A cloned state directory is a cloned identity.** `state.sqlite` holds the
   device secret key and the network secrets. Copying it copies the participant.
-  Restoring an old backup rolls the agent's state back, which — once signed
-  records exist — can resurrect revoked information or replay stale versions.
+  Restoring an old backup rolls the agent's state back and can replay stale
+  versions until it meets a replica holding the newer signed state. Retained
+  cleanup markers prevent old claims from resurrecting after that merge; they
+  cannot help if every copy of a marker has been lost.
 - **A compromised host.** The secret is on disk to survive restarts. File
   permissions are owner-only where the platform supports it, and the state
   directory takes an ownership lock, but neither defends against a user who can