Support RUST_LOG, enable UPnP portmapper and add NAT traversal logging

Previously, `tsunagi-cli` configured clap with a fixed default_value and
`env = "TSUNAGI_LOG"`, which caused `RUST_LOG` to be completely ignored.
In addition, `iroh` was built with `default-features = false` and lacked the
`"portmapper"` feature, which caused it to compile a stub client and disable
UPnP-IGD, PCP, and NAT-PMP port mapping. Furthermore, there was no visibility
into STUN address discovery, UPnP port mapping status, or QUIC transmission path
switching (hole punch vs relay).

Key improvements:
- Enable the `"portmapper"` feature on `iroh` in workspace Cargo.toml, pulling in
  `portmapper` and `igd-next` to actively request port mappings on local routers.
- In `crates/tsunagi-cli/src/main.rs`, support `RUST_LOG` with fallback precedence
  (`--log` > `TSUNAGI_LOG` > `RUST_LOG` > default filter). When debug or trace
  filtering is active, automatically enable `.with_target(true)` so users can see
  which subsystem emitted the log (e.g., portmapper, iroh::magicsock, tsunagi::net).
- In `crates/tsunagi/src/net.rs`, monitor `endpoint.watch_addr()` and
  `endpoint.home_relay_status()`. Emit `DEBUG` logs for local socket binding,
  discovered STUN reflexive/UPnP addresses, and relay connections. If no direct
  addresses are discovered after probe grace, emit a concise `WARN`.
- In `crates/tsunagi/src/agent/session.rs`, monitor connection `path_events()`
  to log when transmission paths are opened, closed, or selected (switching between
  relay and direct hole-punched paths), with bounded task winding down on shutdown.

Validation on Linux:
- cargo fmt --all -- --check passed.
- cargo clippy --locked --workspace --all-targets -- -D warnings passed.
- cargo test --locked --workspace --all-targets passed (349 tests passed).
- Live verification with `RUST_LOG=debug` confirmed SSDP broadcast to 239.255.255.250:1900,
  portmapper logs, STUN QAD discovery, and clean relay shutdown.

Cargo.toml

@@ -27,7 +27,7 @@ expect_used = "warn"
 panic = "warn"
 
 [workspace.dependencies]
-iroh = { version = "1.2", default-features = false, features = ["tls-ring"] }
+iroh = { version = "1.2", default-features = false, features = ["tls-ring", "portmapper"] }
 tokio = { version = "1.53", features = ["rt", "rt-multi-thread", "sync", "time", "macros"] }
 serde = { version = "1.0", features = ["derive"] }
 postcard = { version = "1.1", default-features = false, features = ["use-std"] }