build(android): debug APKs ship release-profile rust

Cargo's debug profile is not "slower" for this library — it is unusable, and
it invalidated every on-device performance test to date: RustCrypto's AES-GCM
compiles to generic-array iterator closures with per-byte precondition checks
instead of ARMv8 hardware AES. Profiled live on a phone (simpleperf, 62k
samples): ~800 µs of user CPU per 1.4 KB packet — the receive pump pinned
above a full core yet only draining ~1,400 pkt/s of a 1,775 pkt/s (20 Mbps)
stream, 2.3 MB standing in the kernel socket buffer, the latency-bound flush
firing every 2 s forever. With release rust in the same debug APK: pump at
~12 % of a core, socket queue zero, no flushes, 2800x1260@120 streaming clean.

preDebugBuild now depends on cargoNdkRelease; `-PrustDebug` opts back into a
debug-profile native build for sessions that actually step through Rust.
Kotlin debuggability is unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/android/kit/build.gradle.kts

@@ -110,8 +110,18 @@ afterEvaluate {
     // screenshot unit tests render Compose on the JVM and never load libpunktfunk_android.so), so
     // CI/local screenshot runs don't need the Rust toolchain or NDK. The native build stays wired
     // for every normal APK/AAR build.
+    //
+    // DEBUG APKs SHIP RELEASE RUST. Cargo's debug profile is not "a bit slower" for this library —
+    // it is unusable: the AES-GCM data-plane decrypt runs through generic-array iterator closures
+    // with per-byte UB checks instead of ARMv8 hardware AES. Profiled live on a phone (simpleperf):
+    // ~800 µs of user CPU per 1.4 KB packet, the receive pump pinned over a full core yet unable to
+    // drain a 20 Mbps stream — every debug-APK on-device test was silently benchmarking unoptimized
+    // crypto, not the streaming pipeline. Kotlin debuggability is untouched (the APK is still a
+    // debug build); only the cargo profile changes. `-PrustDebug` restores a debug-profile native
+    // build for the rare session that actually steps through Rust.
     if (!project.hasProperty("skipRustBuild")) {
-        tasks.named("preDebugBuild").configure { dependsOn(cargoNdkDebug) }
+        val debugRust = if (project.hasProperty("rustDebug")) cargoNdkDebug else cargoNdkRelease
+        tasks.named("preDebugBuild").configure { dependsOn(debugRust) }
         tasks.named("preReleaseBuild").configure { dependsOn(cargoNdkRelease) }
     }
 }