Improve fast deployment story when app is installed manually (missing assemblies → confusing crash)

## Summary

When Fast Deployment is enabled (the default for `Debug` builds), `dotnet build` produces an `.apk` that **does not contain the managed `.dll` assemblies** — they're pushed separately to the device's override directory during the `Install` target via `adb push`/`run-as`.

This trips up a common manual workflow that both humans and AI agents reach for:

```sh
dotnet build
adb install path/to/my.apk
```

The app installs fine, launches, and then **immediately crashes** because no assemblies are on the device. The failure is opaque — nothing tells you that you skipped Fast Deployment and should have used `dotnet build -t:Install` (or disabled Fast Deployment).

## Current behavior

At runtime we abort with a `log_fatal`. The messages today (from `src/native/.../monodroid-glue.cc` and the CLR host) look roughly like:

```
No assemblies found in '' ... Assuming this is part of Fast Deployment. Exiting...
FastDev assembly 'Foo.dll' not found.
```

Even in `DEBUG` this doesn't clearly tell the user *what to do next*, and in a `logcat` flood it's easy to miss. There's no in-app signal at all — just a crash.

## Proposed improvements

1. **In-app fallback activity** — When the app detects it launched with no deployed assemblies (empty/missing override dir under Fast Deployment), show a plain **Java-based `Activity`** (no managed code required, since managed code can't run) that says something like:

> ⚠️ No .NET assemblies were deployed.
> This APK was built with Fast Deployment, which keeps assemblies out of the `.apk`.
> Run `dotnet build -t:Install` to deploy properly, or build with `-p:EmbedAssembliesIntoApk=true` / a `Release` build to produce a self-contained `.apk`.

2. **Better logcat diagnostics** — Upgrade the fatal message to be explicit and greppable, aimed at helping an AI/human self-correct, e.g.:

> FATAL: No .NET assemblies found on device. This app was built with Fast Deployment; `adb install` alone does not deploy assemblies. Use `dotnet build -t:Install` or set `-p:EmbedAssembliesIntoApk=true`.

## Notes / open questions

- The Java activity approach is attractive because when assemblies are missing, the managed runtime can't start — so the guidance must live in Java/native.
- Should the fallback activity be Debug-only? (It should never ship in `Release`, where assemblies are always embedded.)
- We may be able to detect this specific condition (Fast Deployment build + empty override dir) distinctly from the "compressed assemblies in APK" failure, so the message is precise.

Start with the missing-assembly handling in src/native/.../monodroid-glue.cc and the CLR host, then trace how the empty or missing Fast Deployment override directory is detected. Define the Java fallback Activity and improve the fatal logcat diagnostic so both explain the skipped Install target and the available build options; verify that Release behavior remains unchanged.

Written by the indexing model from the issue text.