Add build-and-test suite (in-app self-test + publish gate); fix release-zip missing sounds

The test suite, modelled on Andre's Sensor Readout (an in-app self-test + a build
script), runnable as one step before every publish.

Part 1 - in-app multi-step self-test (SelfTest.cs), run by --selftest:
  audio round-trip (PCM + Opus over localhost, dedicated test port so it never
  clashes with a running instance), encryption right/wrong-password + fingerprint,
  packet framing + malformed rejection, client<->server wire-format compatibility,
  settings save/reload, profile save/reload (temp folder), diagnostics-report
  privacy (never leaks a password), and bundled-resources present. Each step is
  timed and reported PASS/FAIL/SKIP; exit 0 only if nothing failed. Replaces the
  old single-shot --selftest. RunDiagnostics refactored to expose
  BuildDiagnosticsReport(AppConfig) for the privacy step.

Part 2 - run-tests.ps1: builds, then checks the package (sounds, readme, native
  opus, framework-dependent, dll version == csproj), the About-box changelog, the
  client/server wire contract (relay magic/version/port still match RemPacket),
  the CLI surface, and runs --selftest. build-release.ps1 now runs this gate first
  and aborts the release if it fails.

Bug caught + fixed: the published release zip carried ZERO cue sounds (startup
  sound + connect/disconnect/etc.) - MSBuild's incremental Content-copy marker
  skipped sounds\ on a fresh publish. Added an AfterTargets=Publish copy in the
  csproj that lands every cue WAV in the published sounds\ folder regardless of
  the marker. Verified: a staging publish now contains all 9 cue WAVs.

Manual/help: --selftest description updated (readme.html + MANUAL.md).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

readme.html

@@ -1154,7 +1154,7 @@ Use whatever key combinations you prefer (for example Ctrl+Shift+Up / Ctrl+Shift
 <tr><td><code>--help</code> or <code>-h</code></td><td>Lists every option, the same as this section in short form.</td></tr>
 <tr><td><code>--version</code></td><td>Prints which version of RemSound is installed, for example &ldquo;RemSound 3.9&rdquo;.</td></tr>
 <tr><td><code>--devices</code></td><td>Lists every microphone and line-in, every speaker and headphone output, and every ASIO driver on the machine &mdash; each with its sample rate, channel count and the exact device id RemSound uses internally. This is the quickest way to confirm an interface is actually present and seen by Windows.</td></tr>
-<tr><td><code>--selftest</code></td><td>Runs a complete round-trip on the machine on its own: it captures sound, encodes it, sends it across the network layer to itself, receives it and decodes it, then reports <strong>PASS</strong> or <strong>FAIL</strong>. No sound is played out, so it is safe to run silently. A PASS proves capture, encoding, the network code and decoding are all working on that computer. Add <code>--opus</code> to test the Opus codec path, or <code>--seconds N</code> to run it for longer than the default.</td></tr>
+<tr><td><code>--selftest</code></td><td>Runs RemSound's built-in self-test and reports <strong>PASS</strong> or <strong>FAIL</strong>. It works through a list of named checks: a full audio round-trip on the machine on its own (capture &rarr; encode &rarr; send across the network layer to itself &rarr; receive &rarr; decode, for both quality settings), the audio encryption, the network packet format, saving and reloading settings and a profile, that a diagnostics report never leaks a password, and that the bundled sounds and manual are present. No sound is played out, so it is safe to run silently. Add <code>--seconds N</code> to make the audio part run for longer than the default.</td></tr>
 <tr><td><code>--diagnostics</code></td><td>Writes a single plain-text report file holding the version, the operating system, the current settings, the list of profiles, the full device list, a check of the Windows microphone-privacy permission, and the tail of the most recent log. With no path it saves into the <strong>user settings and logs</strong> folder and prints where it put it; you can also give a path, for example <code>--diagnostics C:\Users\you\Desktop\report.txt</code>. This is the file to send when asking for help &mdash; it answers most questions in one go.</td></tr>
 </table>