dev-updater: build an app on the machine, install it on the phone

A Rust backend that discovers Android projects under configured roots,
builds one on request, and serves the APK over pinned TLS on a WireGuard
interface; an Android client that lists what is buildable, watches a build,
and installs the result. Enrolment carries the token and the CA, so the
phone trusts exactly the machine that issued it and nothing else.

`AGENTS.md` is the working guide and `README.md` the configuration
reference. The shared tunnel-and-TLS code lives in `vendor/wg-app-link`,
which ai-app uses too.

History before this point was squashed away, and a stale `config.json` went
with it: nothing had read that file since the config moved to RON outside
the checkout, and what it still held was one machine's absolute paths and
the names of projects on it.

test-projects/two-variants/.dev-updater.ron

@@ -0,0 +1,15 @@
+// A test project for Dev Updater, not a real app. See ../README.md.
+//
+// Builds the same app twice under different variant directories, which is
+// the only thing this one is for: with more than one build present the
+// phone gets a variant picker, `resolve_apk` has something to fall back
+// from when a chosen variant is deleted, and the manifest has to report the
+// size of the one actually being served.
+label: "Test: Two Variants",
+
+components: [
+    Apk(
+        name: "app",
+        build: "./build.sh",
+    ),
+],