Route native logging through the log crate

[ruccho]

Closes #170.

What

Replaces every println!/eprintln! in the native code (47 call sites across six crates) with the
log facade plus a single log::Log implementation in unienc_c, and adds the Unity-side controls
for the resulting level.

Sinks

Per record, first match wins:

1. IUnityLog — unity feature on and UnityPluginLoad has run. Android included: Unity's log
   destination on Android already is logcat, so this gives up no destination and adds the Editor
   console and the console of an Editor attached to a development player.
2. __android_log_write under the unienc tag — Android, outside the window above.
3. stdout/stderr — NuGet package, CLI tests, wasm.

Log level

Defaults to Warn for release builds of the native library and Debug for debug builds.
unienc_set_log_level, exposed as UniEnc.NativeLogging.SetLevel, changes it at runtime. Several
previously commented-out per-frame dumps are now live at trace!.

Preferences > UniEnc drives it:

• Applies to the Editor's own copy of the plugin, and is pushed to connected development players
  over the player connection, so a running player picks up a change.
• Bake Into Player Builds (off by default) writes the level into the build as a Resources
  ScriptableObject applied at BeforeSplashScreen — the only way to catch the records a session
  emits while starting up, and the only way a release build gets a level at all.
• Default stores nothing and calls nothing, leaving the plugin unloaded and replaceable on disk.

Breaking changes

Windows and Linux Unity binaries change. build-unienc.yml now passes -F unity,mimalloc there
instead of -F mimalloc alone — without it those builds would silently println instead of using
IUnityLog. They now export UnityPluginLoad/UnityPluginUnload and Unity calls them, which also
runs mimalloc::unity::init and routes mimalloc through IUnityMemoryManager. That path is
exercised on macOS, iOS and Android but not previously on these two. Please validate on real
player builds for both before release.

Making -F unity compile there needed empty UnityPlugin impls for Media Foundation, FFmpeg and
WebCodecs, each behind its own unity feature so unity-native-plugin stays out of the NuGet
builds. wasm keeps --no-default-features.

set_stdout_redirect is removed (Android). It dup2'd stdout/stderr process-wide from
JNI_OnLoad, re-tagging every library's output in the process as unienc. Native code that reached
logcat only through that redirect now needs its own logging.

UniEnc's own logcat tag changes to Unity's under the unity feature. The message body keeps its
[unienc] prefix, so adb logcat | grep '\[unienc\]' replaces adb logcat -s unienc.

Worth knowing: Unity captures a managed stack trace per record in a development build (None in
release), so high-volume trace! on a device is not free. The mitigation,
Application.SetStackTraceLogType, is application-global and so not this package's call.

Verification

cargo check -p unienc_c --features unity passes for aarch64-apple-darwin, aarch64-apple-ios,
aarch64-linux-android, x86_64-pc-windows-msvc and x86_64-unknown-linux-gnu; without the
feature it passes for macOS, Windows, Linux, wasm and Android. cargo fmt --check is clean and
Cargo.lock gains no new dependencies, so THIRD-PARTY-NOTICES.md is unchanged.

Exercised in a 6000.2.13f1 Editor on Windows against the User Interfaces sample: records reach the
console at the right LogType (including from Media Foundation's own worker threads) with nothing
duplicated to the stdio fallback, each level filters as expected, the preference survives a domain
reload, a development player applies the level over the connection both at startup and on change,
and a release player with the level baked in logs the full MFT selection that the connection path
cannot reach.

Not verified on a real Android device — the sink order there is compile-checked only. Also
unverified: the Apple and WebCodecs backends.

Native binaries need a build-unienc.yml run before this is usable from the package.