Rename the upper module to ipv6tun

The upper module holds the TUN adapter, the .fips DNS responder, ICMPv6
generation, TCP MSS clamping and the IPv6 shim codec: the host-side IPv6
plane. Name the directory for what it is, so host-networking code has
one obvious home as more of it is gathered there.

This is a pure directory move. No moved file changes, and a
`pub use ipv6tun as upper;` alias in lib.rs keeps every crate::upper::
and fips::upper:: path resolving, so no consumer changes either.

Tracing targets follow module paths, so fips::upper::* becomes
fips::ipv6tun::*. Update the RUST_LOG example in the MTU diagnosis guide
and note the renamed targets in the changelog.

CHANGELOG.md

@@ -328,6 +328,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   binder tearing down and rebinding every second while teardown silently
   declined to abort anything.
 
+- Tracing targets of the TUN adapter, DNS responder and ICMPv6 code move from
+  `fips::upper::*` to `fips::ipv6tun::*`, because the module they log from is
+  now `ipv6tun` (for example `fips::upper::tun` becomes `fips::ipv6tun::tun`).
+  An existing `RUST_LOG` filter naming an old target still parses and simply
+  stops matching, so the symptom is missing log lines rather than an error.
+  Update `RUST_LOG` filters, journal-watch recipes and any log-scraping alert
+  accordingly. The library path `fips::upper` still resolves.
+
 #### Packaging (Debian)
 
 - An upgrade of the `.deb` now reapplies the firewall ruleset in place. Until

docs/how-to/diagnose-mtu-issues.md

@@ -87,7 +87,7 @@ fully-qualified module paths under the `fips` crate.
 sudo systemctl edit fips
 # Add:
 # [Service]
-# Environment=RUST_LOG=info,fips::upper::tun=trace,fips::node::handlers::mmp=debug
+# Environment=RUST_LOG=info,fips::ipv6tun::tun=trace,fips::node::handlers::mmp=debug
 sudo systemctl restart fips
 sudo journalctl -u fips -f
 ```

src/lib.rs

@@ -20,6 +20,7 @@ pub mod identity;
 // modules that follow.
 #[macro_use]
 pub(crate) mod instr;
+pub mod ipv6tun;
 pub mod mdns;
 pub mod native;
 pub mod node;
@@ -32,10 +33,13 @@ pub(crate) mod proto;
 pub(crate) mod testutil;
 mod time;
 pub mod transport;
-pub mod upper;
 pub mod utils;
 pub mod version;
 
+// `upper` is the former name of `ipv6tun`; the alias keeps `crate::upper::`
+// and `fips::upper::` paths resolving.
+pub use ipv6tun as upper;
+
 // Re-export identity types
 pub use identity::{
     AuthChallenge, AuthResponse, FipsAddress, Identity, IdentityError, NodeAddr, PeerIdentity,