Give DodoSSH a phone, and a shared shell for both heads to drive

The Android head from docs/android-port.md, taken as far as its step 6.

Step 3, the spike, is answered and its throwaway screen is gone: libsodium.so and
libe_sqlite3.so are both in the arm64 APK, so NSec resolves its native half on Android
despite shipping no Android build, and the local cache opens. Two findings the audit
could not have had: Avalonia.Controls.WebView only ships net10.0-android36.0, which
settles the open "which Android versions" question at targetSdk 36; and Android has
blocked cleartext HTTP since API 28, so the terminal renderer needs a network security
config scoped to 127.0.0.1 or the WebView loads nothing.

DodoSSH.Client.Shell is new and is why the phone can exist: the view models, the terminal
renderer files and the palette moved there so both heads drive one state machine and draw
from one set of tokens. The desktop head is otherwise untouched and its 144 tests still
pass.

The platform pieces behind interfaces that already existed: the profile directory from
filesDir, a device key wrapped by a StrongBox-backed key that a fingerprint releases, and
a foreground service so a shell outliving a vault lock stays true on a platform that
stops backgrounded processes.

Sign-in is deliberately absent rather than approximated. It needs an app link, because
reusing the desktop loopback listener is the attack RFC 8252 section 8.3 names.

src/DodoSSH.Client.Shell/WebAssets/vendor/README.md

@@ -0,0 +1,28 @@
+# Vendored third-party assets
+
+Committed rather than fetched at build time, so a clean clone builds with the .NET SDK alone — no
+node, no npm, no esbuild step. The trade-off is that upgrades are a manual re-download, which is
+exactly why the provenance is written down here.
+
+All of the following are MIT licensed, the same licence as this repository.
+
+| File | Package | Version | Source |
+| --- | --- | --- | --- |
+| `xterm.js` | `@xterm/xterm` | 6.0.0 | `https://unpkg.com/@xterm/xterm@6.0.0/lib/xterm.js` |
+| `xterm.css` | `@xterm/xterm` | 6.0.0 | `https://unpkg.com/@xterm/xterm@6.0.0/css/xterm.css` |
+| `addon-fit.js` | `@xterm/addon-fit` | 0.11.0 | `https://unpkg.com/@xterm/addon-fit@0.11.0/lib/addon-fit.js` |
+| `addon-webgl.js` | `@xterm/addon-webgl` | 0.19.0 | `https://unpkg.com/@xterm/addon-webgl@0.19.0/lib/addon-webgl.js` |
+
+Copyright (c) 2017 The xterm.js authors. See <https://github.com/xtermjs/xterm.js> for the full
+licence text.
+
+These are the UMD builds, so they attach to `globalThis`: `Terminal`, `FitAddon.FitAddon` and
+`WebglAddon.WebglAddon`. `terminal.js` depends on those exact names, so check them after any upgrade
+— a bundle that switched to ES modules would load without error and leave `Terminal` undefined.
+
+## Upgrading
+
+1. Download the four files at the new versions and update the table above.
+2. Confirm the UMD global names are unchanged.
+3. Run the terminal suite, then start the app and open a session: nothing here is covered by the
+   .NET tests, because the renderer they exercise is a `ClientWebSocket` standing in for this page.