v3.4 refinements: WASAPI drift correction, startup-dialog sequencing, quick-switch polish, docs

Builds on the v3.4 freeze (dd70613) with the fixes and tuning from live testing,
plus the v3.4 documentation pass.

Audio (receiver):
- Per-device WASAPI drift correction in MultiOutputPlayout. A pull-side resampler
  (mirroring SessionPlayout's proven corrector) holds each output device's buffer at
  a fixed low depth, cancelling the slow clock drift that made WASAPI peers "lag
  apart" over long sessions. Feed-forward clock-ratio measurement plus a gentle
  depth-restoring term; the first measurement window is discarded because WASAPI
  start-up priming poisons it. ASIO already self-corrected; this brings WASAPI level.
- Output device buffer requested at 5 ms (WASAPI clamps it up to the device's minimum
  period, ~10 ms) instead of 15 ms, since the corrector keeps it fed — a free saving.

UI / accessibility (MainForm, Program):
- Startup notices (what's-new About box, Realtek warning) now run one at a time via a
  single sequence instead of separate BeginInvokes, so they no longer stack into
  nested modals that couldn't be closed. The loading splash is skipped for a
  tray-bound quick switch.
- Quick profile switch keeps RemSound in the tray if it was there, and plays the
  switch cue immediately on click.
- Profile-switch cue now plays on click for every switch path (recent menu, quick
  switch, File > Open) and no longer on a fresh start into the first profile. It was
  also previously dead on the rebuilt form (pendingProfile was nulled first).
- Realtek ASIO toggle's accessible name now reads "Enable"/"Disable" to match the
  visible text, instead of "Toggle" (screen reader read the wrong word).

Docs (plain English):
- RELEASE_NOTES.md: v3.4 entry.
- About dialog: v3.4 "what's new".
- readme.html (the canonical bundled manual): quick switch, the hotkey read-outs, the
  new profile-menu-open cue, Realtek auto-detect/disable, and the config-folder path.
- MANUAL.md regenerated from readme.html via sync-manual.py so the two stay in sync.

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

src/RemSound.App/Program.cs

@@ -186,7 +186,11 @@ internal static class Program
                 // MainForm construction. Show a "Loading audio driver" splash — on its own
                 // thread, so it stays painted while this thread is busy — so startup doesn't
                 // look hung. No-op for WASAPI-only profiles (construction is near-instant).
-                var splash = AsioLoadingSplash.StartIfNeeded(profile);
+                // Skip the loading splash when this rebuild is a quick-profile-switch that's
+                // staying in the tray — popping a splash up in front of the user's current app
+                // defeats the point of keeping RemSound minimised, and the switch cue already
+                // gave them feedback. Normal launches and visible switches still show it.
+                var splash = MainForm.startNextInstanceMinimized ? null : AsioLoadingSplash.StartIfNeeded(profile);
                 using var form = new MainForm(store, profile, title, nextPath);
                 // Expose the live window to the single-instance activation callback (a second
                 // copy choosing "switch to the running copy" signals us to surface this form).