feat(mini): add compact native EasyTier client (#2479)

Add a native EasyTier proof-of-concept binary with TCP and UDP
transports, TUN, UDP hole punching, AES-GCM, and a read-only RPC
portal.

Introduce a release-derived mini profile and musl linker policy so
x86_64, big-endian MIPS, and little-endian MIPS stay below the strict
5,000,000-byte target without UPX.

easytier-contrib/easytier-mini/README.md

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+# easytier-mini
+
+`easytier-mini` is a native EasyTier POC binary. It shares EasyTier's TOML
+configuration model, peer protocol, TCP/UDP tunnel implementations, TUN,
+dynamic IPv4 allocation, the smoltcp userspace path and STUN/UDP hole-punching
+core with the full binary. It includes AES-GCM so its default encryption
+setting interoperates with the full binary's default configuration.
+
+Build it with:
+
+```sh
+cargo build --release -p easytier-mini
+```
+
+For the static size target used by this POC:
+
+```sh
+cargo build --profile mini --target x86_64-unknown-linux-musl -p easytier-mini
+```
+
+MIPS targets use the repository's existing musl-cross toolchains. The helper
+builds the standard library for size, applies immediate-abort only to the mini
+MIPS target graph, and can build either or both byte orders:
+
+```sh
+./easytier-contrib/easytier-mini/build-mips.sh all
+./easytier-contrib/easytier-mini/build-mips.sh mips
+./easytier-contrib/easytier-mini/build-mips.sh mipsel
+```
+
+The `mini` profile derives from `release` and applies `opt-level=z` to the
+entire compact binary dependency graph. Full EasyTier release builds retain
+their normal `opt-level=3` profile. The musl builds use a mini-only static
+linker policy to stay below 5,000,000 bytes on x86-64 and 5,500,000 bytes on
+MIPS without UPX or another executable compressor. The compact x86-64 linker
+policy retains static PIE, packs relative relocations and folds identical code.
+MIPS builds omit standard-library backtrace support and use immediate abort;
+normal workspace MIPS builds are not affected. Compact linker policies omit
+unwind tables.
+
+Start it with a normal EasyTier TOML file:
+
+```sh
+easytier-mini --config mini.toml
+```
+
+`-c` is accepted as the short form of `--config`.
+
+Start it as an EasyTier Web managed node with a complete config-server URL:
+
+```sh
+easytier-mini --config-server udp://config-server.easytier.cn:22020/TOKEN
+```
+
+`--machine-id`, `--hostname`, and `--secure-mode` match the full client's Web
+identity and transport options. `--config` and `--config-server` may be used
+together: the local instance remains static while Web-owned instances are
+created, updated, retained, and deleted independently.
+
+The node also exposes the native EasyTier management RPC protocol on
+`127.0.0.1:15888`, so the full `easytier-cli` can inspect it:
+
+```sh
+easytier-cli node info
+easytier-cli peer
+easytier-cli route
+easytier-cli connector list
+```
+
+For example:
+
+```toml
+instance_name = "mini"
+ipv4 = "10.147.0.2"
+listeners = ["tcp://0.0.0.0:11010", "udp://0.0.0.0:11010"]
+
+[network_identity]
+network_name = "mini-poc"
+network_secret = "change-me"
+
+[[peer]]
+uri = "tcp://example.net:11010"
+```
+
+Local TOML and Web configuration both retain the complete authoritative model.
+The compact runtime silently omits unsupported capabilities while normalizing
+that model into live runtime state. EasyTier Web therefore sees every accepted
+configuration value unchanged and its consistency checks converge. This also
+applies to hot patches: for example, a port-forward patch remains visible to
+the controller while no port-forward service starts in mini. ChaCha20 falls
+back to AES-GCM rather than plaintext.
+
+The compact runtime supports `tcp://` and `udp://` listener, mapped-listener
+and peer URLs. `no_tun = true` runs through smoltcp without an OS TUN device,
+and `dhcp = true` allocates the virtual IPv4 address dynamically.
+
+The mini feature set keeps STUN collection, UDP hole punching, Web heartbeats,
+Web instance lifecycle management and the config hot-patch RPC. It omits TCP
+hole punching, endpoint discovery (`http://`, `https://`, `txt://` and
+`srv://` peers), protobuf reflection, logger control and the rest of the full
+management surface. Unsupported connector URLs are accepted as no-ops. Its
+local RPC surface remains read-only for node, peer, route and connector
+queries. OSPF route messages keep their original protobuf wire data, so fields
+added by future EasyTier versions are forwarded without requiring
+`prost-reflect`.
+
+For size, this POC reads one file directly and does not support configuration
+from stdin or `${VAR}` expansion. It omits the process-management event journal,
+while the console logger still reports runtime events such as peer, connection,
+listener, TUN and DHCP changes. The RPC address is currently fixed, so only one
+mini process can use the default portal on a host. The x86-64 musl POC cannot
+provide reliable stack backtraces because its release binary has no unwind
+tables.