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.lock

@@ -234,6 +234,18 @@ version = "1.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1505bd5d3d116872e7271a6d4e16d81d0c8570876c8de68093a09ac269d8aac0"
 
+[[package]]
+name = "attohttpc"
+version = "0.30.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "16e2cdb6d5ed835199484bb92bb8b3edd526effe995c61732580439c1a67e2e9"
+dependencies = [
+ "base64 0.22.1",
+ "http",
+ "log",
+ "url",
+]
+
 [[package]]
 name = "autocfg"
 version = "1.5.1"
@@ -1112,6 +1124,12 @@ dependencies = [
  "spin 0.9.9",
 ]
 
+[[package]]
+name = "fnv"
+version = "1.0.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
+
 [[package]]
 name = "foldhash"
 version = "0.2.0"
@@ -1341,6 +1359,25 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "h2"
+version = "0.4.19"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ef8e5e5a340588f4452631496976cf8636d4a7ecf600239fdc27615d2530bc16"
+dependencies = [
+ "atomic-waker",
+ "bytes",
+ "fnv",
+ "futures-core",
+ "futures-sink",
+ "http",
+ "indexmap",
+ "slab",
+ "tokio",
+ "tokio-util",
+ "tracing",
+]
+
 [[package]]
 name = "hashbrown"
 version = "0.16.1"
@@ -1473,6 +1510,7 @@ dependencies = [
  "bytes",
  "futures-channel",
  "futures-core",
+ "h2",
  "http",
  "http-body",
  "httparse",
@@ -1656,6 +1694,26 @@ dependencies = [
  "icu_properties",
 ]
 
+[[package]]
+name = "igd-next"
+version = "0.17.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "de7238d487a9aff61f81b5ab41c0a841532a115a398b5fa92a2fadd0885e2581"
+dependencies = [
+ "attohttpc",
+ "bytes",
+ "futures",
+ "http",
+ "http-body-util",
+ "hyper",
+ "hyper-util",
+ "log",
+ "rand 0.10.3",
+ "tokio",
+ "url",
+ "xmltree",
+]
+
 [[package]]
 name = "indexmap"
 version = "2.14.2"
@@ -1748,6 +1806,7 @@ dependencies = [
  "papaya",
  "pin-project",
  "portable-atomic",
+ "portmapper",
  "rand 0.10.3",
  "reqwest",
  "rustc-hash",
@@ -2936,6 +2995,35 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "portmapper"
+version = "0.19.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca97242f016e090a25330613bc2382aab65eb22f9149dbe84d8f8e711d49a530"
+dependencies = [
+ "base64 0.22.1",
+ "bytes",
+ "derive_more",
+ "hyper-util",
+ "igd-next",
+ "iroh-metrics",
+ "libc",
+ "n0-error",
+ "n0-future",
+ "netwatch",
+ "num_enum",
+ "rand 0.10.3",
+ "serde",
+ "smallvec",
+ "socket2",
+ "time",
+ "tokio",
+ "tokio-util",
+ "tower-layer",
+ "tracing",
+ "url",
+]
+
 [[package]]
 name = "postcard"
 version = "1.1.3"
@@ -4138,6 +4226,7 @@ dependencies = [
  "thiserror 2.0.20",
  "tld",
  "tokio",
+ "tokio-stream",
  "tracing",
  "tracing-subscriber",
  "tsunagi",
@@ -4876,6 +4965,21 @@ dependencies = [
  "time",
 ]
 
+[[package]]
+name = "xml-rs"
+version = "0.8.29"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e450f9b2ed1dff33c94c12589a87338689467b9c4f5d8a5710bd09a847d2c8a7"
+
+[[package]]
+name = "xmltree"
+version = "0.10.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d7d8a75eaf6557bb84a65ace8609883db44a29951042ada9b393151532e41fcb"
+dependencies = [
+ "xml-rs",
+]
+
 [[package]]
 name = "yasna"
 version = "0.6.0"