Test suite: richer diagnostics, headless accessibility audit, perf/leak sanity

Andre's three "bigger ideas" from RemSound-smoke-test-agent-brief.md:

- Richer diagnostics: --diagnostics now includes a live localhost audio self-check
  (PCM + Opus, with packet/underrun/drop/buffer/latency counters), the most recent
  session snapshot parsed from the log (codec, send/receive state, buffer, drops,
  heartbeat), and a recent-warnings/errors digest from the log. BuildDiagnosticsReport
  gained a runLiveAudioProbe flag so the self-test's privacy check stays fast.

- Headless accessibility audit: new --selftest step constructs the dialogs that can be
  built without hardware (Startup behaviour, Recording settings, Preferences) and checks
  every actionable control announces a name and that Alt-key mnemonics are unique within
  a container. MainForm is out of scope (its constructor opens audio/hotkeys/sockets).
  Dialogs that won't construct are skipped, not failed. Currently audits 3, no violations.

- Perf/leak sanity: new --perftest command runs several audio-loopback cycles and reports
  whether handle/memory/thread counts stay bounded (handles ratcheting up cycle-on-cycle is
  the leak fingerprint, given RemSound's handle-leak history). Lenient thresholds; logs the
  numbers for build-to-build comparison. Wired into run-tests.ps1.

- Shared AudioLoopback helper (used by the self-test, diagnostics and perf test) so all
  three exercise the identical real capture/encode/network/decode path on test port 47929.

- csproj: the four previously-unconditional cue Content items are now Exists-guarded like
  the rest, so a mid-edit sounds\ folder doesn't break the dev build; the gate still
  enforces the required cues before release.

Help + manual updated (--perftest, --smoke-test, --config-dir, richer --selftest).

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

readme.html

@@ -1155,7 +1155,8 @@ Use whatever key combinations you prefer (for example Ctrl+Shift+Up / Ctrl+Shift
 <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><br>(or <code>--smoke-test</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>
+<tr><td><code>--perftest</code><br>[<code>--seconds N</code>]</td><td>Runs the audio path through several short cycles and reports whether RemSound's handle, memory and thread use stays bounded &mdash; a quick check for resource leaks. Prints the numbers each cycle so two builds can be compared. No sound is played out.</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, a quick live audio self-check, the most recent session snapshot and recent warnings from the log, 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>
 
 <h3>Options that change a setting or control a running copy, then exit</h3>