feat(65): the replay ring — encoded fragments in RAM, capped by bytes

Slice 1 of TASK 65, pure memory, no encoder change yet.

A 1080p BGRA frame is 8.3 MB, so fifteen seconds of raw composite is ~7.4 GB — the 15 MB figure in the
spec is the *compressed* stream. So the ring holds fragmented-MP4 bytes that ffmpeg will write to a
pipe, and a save is init-segment + surviving fragments in order.

Two rules the tests pin: the cap is by **bytes** (bitrate moves; seconds do not), and the newest
fragments always win, because a clip that ends at the keypress is the entire point. The init segment does
not rotate and cannot be reused across encoder starts, so `Clear` drops it too — an MP4 without its moov
is noise.

Next slice: ffmpeg writes the buffer output to stdout while `-stats` stays on stderr, and the pump feeds
this ring. Not done here on purpose — an encoder change is the riskiest edit in the app and deserves its
own commit and test.

Services/ClipRing.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Collections.Generic;
+
+namespace ytLive.Services;
+
+/// <summary>
+/// TASK 65: the replay buffer's memory. ffmpeg writes fragmented MP4 to a pipe; the app keeps the last
+/// N seconds of those fragments here and hands them back whole.
+///
+/// <para><b>Why encoded bytes and not frames:</b> a 1080p BGRA composite frame is 8.3 MB, so fifteen
+/// seconds at 60 fps is ~7.4 GB of raw frames. The 15 MB figure is the compressed stream, and only that
+/// fits in RAM. <b>Why whole fragments:</b> a fragment is the smallest thing that can be replayed, so the
+/// cap is a byte cap and the clip may be slightly shorter than the requested seconds — never corrupt.</para>
+/// </summary>
+public sealed class ClipRing
+{
+    private readonly long _capBytes;
+    private readonly Queue<byte[]> _fragments = new();
+    private byte[]? _init;
+    private long _bytes;
+
+    public ClipRing(long capBytes)
+    {
+        if (capBytes <= 0) throw new ArgumentOutOfRangeException(nameof(capBytes));
+        _capBytes = capBytes;
+    }
+
+    /// <summary>The stream's initialisation segment (ftyp + moov). Every replayable file needs it, and
+    /// it does not rotate — it is written once per encoder start.</summary>
+    public void SetInit(byte[] init)
+    {
+        _init = init ?? throw new ArgumentNullException(nameof(init));
+        RecomputeBytes();
+        Trim();
+    }
+
+    public void Append(byte[] fragment)
+    {
+        if (fragment is null || fragment.Length == 0) return;
+        _fragments.Enqueue(fragment);
+        _bytes += fragment.Length;
+        Trim();
+    }
+
+    public long Bytes => _bytes;
+    public int FragmentCount => _fragments.Count;
+    public bool IsEmpty => _init is null || _fragments.Count == 0;
+
+    /// <summary>The bytes of a playable clip, init first. Empty array when nothing has been buffered —
+    /// the caller must not write a zero-byte file (a failed save is better than a file that won't open).</summary>
+    public byte[] Snapshot()
+    {
+        if (IsEmpty) return Array.Empty<byte>();
+        var total = _init!.Length;
+        foreach (var fragment in _fragments) total += fragment.Length;
+
+        var output = new byte[total];
+        var offset = 0;
+        Buffer.BlockCopy(_init, 0, output, offset, _init.Length);
+        offset += _init.Length;
+        foreach (var fragment in _fragments)
+        {
+            Buffer.BlockCopy(fragment, 0, output, offset, fragment.Length);
+            offset += fragment.Length;
+        }
+        return output;
+    }
+
+    public void Clear()
+    {
+        _fragments.Clear();
+        _init = null;
+        _bytes = 0;
+    }
+
+    private void RecomputeBytes()
+    {
+        _bytes = _init?.Length ?? 0;
+        foreach (var fragment in _fragments) _bytes += fragment.Length;
+    }
+
+    /// <summary>Drop oldest fragments until the ring fits. Newest data always wins: a clip that ends at
+    /// the moment the creator pressed the key is the whole point.</summary>
+    private void Trim()
+    {
+        while (_fragments.Count > 0 && _bytes > _capBytes)
+        {
+            _bytes -= _fragments.Dequeue().Length;
+        }
+    }
+}