refactor(daemon): share the output ring-window helper

The embedded daemon and the xmrig manager each kept their own copy of
the console output ring: append new bytes, trim the front to a cap at a
newline boundary, and hand a reader everything since its offset. The two
copies had drifted. The daemon tracked an absolute byte total so a
reader's offset stayed valid across a front-trim, while the miner keyed
its reader off the live buffer size, so a trim could silently rewind the
console feed.

- The append/trim and the since-offset logic now live in one small
  header, outputWindowSince and appendBounded, included by both. The
  trim keeps the newline-boundary cut and the guard that stops a lone
  trailing newline from erasing the whole buffer.
- The xmrig manager now carries the same absolute byte total, so its
  reader resolves against the window the same way and survives a trim
  (and a process restart that clears the buffer but keeps the total).

Tests cover feeding the ring well past the trim point while a reader
polls each chunk (the feed is continuous and never dies at the trim),
the lone-trailing-newline guard, and a cleared buffer with a retained
total.

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

src/daemon/embedded_daemon.h

@@ -16,6 +16,8 @@
 #include <set>
 #include <vector>
 
+#include "output_window.h"
+
 #ifdef _WIN32
 #include <windows.h>
 #else
@@ -171,15 +173,7 @@ public:
     /** getOutputSince() on a buffer holding the LAST buffer.size() of `total` bytes ever appended. */
     static std::string outputSince(const std::string& buffer, size_t total, size_t& offset)
     {
-        const size_t base = total - buffer.size();   // absolute position of buffer[0]
-        if (offset < base) offset = base;            // trimmed before it was read: resume at what's left
-        if (offset >= total) {
-            offset = total;
-            return {};
-        }
-        std::string result = buffer.substr(offset - base);
-        offset = total;
-        return result;
+        return outputWindowSince(buffer, total, offset);
     }
 
     /**