Bump to v2.2.0: Opus native binding, efficiency tidy-up, diag self-meter

Single biggest change: added the Concentus.Native NuGet package. Concentus
2.0+ auto-detects native libopus at runtime and routes encode calls
through it; encoder state lives on the C side and is reused across calls
rather than `new`ing ~15 working buffers per call (Concentus issue #22,
open since 2018). Measured on the desktop test at 15:36:55 — Opus 10 ms
allocation rate dropped from 4,625 KB/s to 108 KB/s, a 97.7% reduction.
Process CPU dropped from 4.7% to 1.6% in the same config. Audio is bit-
for-bit identical (it's literally the same encoder, just better
packaged). `OpusEncoderState.cs` itself unchanged on the call site.

Diagnostic / measurement layer (gated on Enable-logs, zero cost when off):
* ProcessSelfMeter: CPU%, managed heap MB, working set MB, allocation
  rate per second, GC counts per generation
* Per-thread work-time counters: captureMs / sendMs / recvMs / renderMs
  expressed as milliseconds of CPU consumed by each audio thread per
  second
* Inter-packet arrival gap measured at the user-space UDP socket
  (rxNetGapMs) — pinpoints whether arrival jitter is in the network or
  our own dispatch path

Small efficiency wins (each one was small but cumulative):
* deviceRefreshTimer interval 1s -> 3s (item 4)
* WaitHandle array allocations eliminated in MixingEngine.MixLoop and
  MultiOutputPlayout.ProduceLoop (item 6)
* MultiOutputPlayout caches its output-buffer snapshot and only rebuilds
  on SetOutputDevices, instead of rebuilding every 10 ms (item 7)
* HeartbeatService reuses an outbound ping byte[] instead of allocating
  per send (item 14)
* PeerDiscoveryService caches broadcast addresses and invalidates on
  Windows' NetworkChange event instead of walking all NICs every 1.5 s
  (item 16)

Legacy / dead-code removal:
* KeepAlive packet's implementation (struct, enums, writer, reader, size
  constant) — all dead since HeartbeatService landed 2026-05-06. Kept
  the RemPacketType.KeepAlive enum value and silent-drop dispatch for
  wire compat with any pre-2026-05-06 build still in the wild (item 30)
* driftDropFramesTotal / driftRepeatFramesTotal fields and accessors —
  Phase-2 splice corrector relics, never incremented since Phase-4
  resampler design landed; backed five always-zero diag log columns
  (items 34 + 35)
* DriftAccumulator (always returned 0) — same shape, removed alongside
  the driftAcc= column (item 35)
* TakeMaxFanOutCacheBytes / Ms + fanCacheMs column — FanOutSource was
  retired in May (item 36)

Project documentation:
* RemSoundefficiency.md added as the canonical record of the efficiency
  analysis, every item's status, and the measured wins from this round
* Honest item-by-item review of the original 50-item list — several
  items I had sized optimistically in the original analysis turned out
  to be already-done (item 20), already-optimal (item 22), or below
  the meter floor (items 9, 15, 17, 25). Recorded so future passes
  don't re-investigate.

Wire format and audio pipeline unchanged from v1.5 onward — v1.5 through
v2.2 peers interoperate.

src/RemSound.App/AboutDialog.cs

@@ -20,14 +20,57 @@ internal sealed class AboutDialog : Form
     /// updates" path.</summary>
     private const string ReleaseNotes =
         """
-        RemSound v2.1
+        RemSound v2.2
 
-        Automatic router setup for internet streaming, a small
-        notice before background updates install, a "lock this
-        profile" option for users who don't want close prompts,
-        and a fix for the "no sound after the laptop wakes up"
-        problem. No wire-format or audio-pipeline changes —
-        v1.5 through v2.1 peers interoperate.
+        A maintenance release that makes RemSound use less of
+        your computer's CPU and memory, especially when sending
+        with the Opus codec. No new features to learn, no
+        settings have changed, and audio sounds exactly the same.
+        Wire format and audio pipeline are unchanged — every
+        version from v1.5 to v2.2 still talks to every other
+        version cleanly.
+
+        What's lighter on your computer:
+          * Opus sending uses much less memory. RemSound's
+            Opus encoder used to put quite a lot of work on
+            Windows' memory manager — about 4 megabytes per
+            second of "throwaway" memory churn while sending
+            Opus audio. v2.2 ships a native build of the same
+            encoder that does its work in a tighter, faster way.
+            The audio you hear is identical (it really is the
+            same encoder, just packaged better); the memory
+            churn drops by about 97 per cent. On laptops you
+            should see less background CPU when streaming Opus,
+            and longer sessions are less likely to see brief
+            pauses while Windows tidies up memory.
+          * Smaller all-round efficiency tidy-up. A handful of
+            small fixes — RemSound checks the audio-device list
+            less often, reuses some small bits of memory it
+            used to make fresh each time, and skips some
+            paperwork on the receive side when there's nothing
+            to do. Each one is small on its own; together they
+            cut RemSound's everyday memory churn modestly.
+          * Removed some old leftover code that was retired
+            months ago but still lived on as zero-valued
+            columns in the diagnostic log. Same behaviour,
+            cleaner files.
+
+        For people who use the diagnostic logs:
+          * Several new columns. "cpu" shows how much of one
+            CPU core RemSound just used. "memMB" and "wsMB" are
+            its memory footprint. "allocKBps" is the per-second
+            memory-churn rate. "captureMs / sendMs / recvMs /
+            renderMs" show how busy each of the four audio
+            threads is. All of this only writes to the log when
+            Enable logs is ticked; with logs off it costs
+            nothing.
+          * "fanCacheMs", "driftDrop", "driftRep" and "driftAcc"
+            columns have been removed — they were always zero
+            after the playback engine changed in May.
+
+        No bug fixes in v2.2 specifically — everything carried
+        over from v2.1's UPnP, lock-profile, wake-from-sleep
+        and hibernate fixes is still in place.
 
         What's new:
           * Automatic router port opening (UPnP). RemSound can