Add VNC Hextile decoding

Implements RFC 6143 SS7.7.4: rectangles announced as Hextile (type 5)
tile the update into 16x16 blocks, each either raw pixels or a
background fill plus an optional list of foreground/individually-
colored subrects, with background/foreground persisting across tiles
within one rectangle when not re-specified.

The pure byte-decode logic (tile metadata, subrect list) lives in new
src/vnc_pixel_codecs.h/.cpp, kept separate from
VncSessionBackend's wire-sequencing state machine so it's unit-testable
without a socket -- the pattern the plan calls for continuing into the
ZRLE/Tight work still ahead. Adds 5 fake-server tests covering a raw
tile, a background-only solid fill, uncoloured and individually-colored
subrects, and a 4-tile rectangle proving background persistence and
correct tile-cursor wraparound.

Verified against the live TightVNC test server (connect, frame,
cursor, clipboard all still work); that particular server always
chose Raw for the actual framebuffer content during this session, so
Hextile's real-world path isn't independently confirmed live -- the
unit tests are the primary correctness evidence for this phase.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/vnc_pixel_codecs.cpp

@@ -0,0 +1,80 @@
+#include "vnc_pixel_codecs.h"
+
+namespace VncPixelCodecs {
+
+QRgb rgbFromPixelBytes(const uchar* bytes)
+{
+    // Byte order B,G,R,pad -- matches VncSessionBackend's negotiated
+    // SetPixelFormat (32bpp little-endian, R at shift 16 / G at 8 / B at 0).
+    return qRgb(bytes[2], bytes[1], bytes[0]);
+}
+
+int hextileFixedMetaByteCount(quint8 subencoding)
+{
+    int count = 0;
+    if ((subencoding & HextileFlags::kBackgroundSpecified) != 0) {
+        count += 4;
+    }
+    if ((subencoding & HextileFlags::kForegroundSpecified) != 0) {
+        count += 4;
+    }
+    if ((subencoding & HextileFlags::kAnySubrects) != 0) {
+        count += 1;
+    }
+    return count;
+}
+
+int decodeHextileFixedMeta(quint8 subencoding, const QByteArray& data, QRgb* background,
+                           QRgb* foreground)
+{
+    int offset = 0;
+    const auto* bytes = reinterpret_cast<const uchar*>(data.constData());
+
+    if ((subencoding & HextileFlags::kBackgroundSpecified) != 0) {
+        *background = rgbFromPixelBytes(bytes + offset);
+        offset += 4;
+    }
+    if ((subencoding & HextileFlags::kForegroundSpecified) != 0) {
+        *foreground = rgbFromPixelBytes(bytes + offset);
+        offset += 4;
+    }
+    if ((subencoding & HextileFlags::kAnySubrects) != 0) {
+        return static_cast<int>(static_cast<quint8>(data.at(offset)));
+    }
+    return 0;
+}
+
+int hextileSubrectByteCount(bool coloured, int subrectCount)
+{
+    return subrectCount * (coloured ? 6 : 2);
+}
+
+QVector<HextileSubrect> decodeHextileSubrects(bool coloured, int subrectCount,
+                                              const QByteArray& data, QRgb foreground)
+{
+    QVector<HextileSubrect> subrects;
+    subrects.reserve(subrectCount);
+
+    const auto* bytes = reinterpret_cast<const uchar*>(data.constData());
+    const int stride = coloured ? 6 : 2;
+    for (int i = 0; i < subrectCount; ++i) {
+        const uchar* entry = bytes + (i * stride);
+        QRgb color = foreground;
+        int fieldOffset = 0;
+        if (coloured) {
+            color = rgbFromPixelBytes(entry);
+            fieldOffset = 4;
+        }
+        const uchar xy = entry[fieldOffset];
+        const uchar wh = entry[fieldOffset + 1];
+        // High nibble = x (or width-1), low nibble = y (or height-1).
+        const int x = (xy >> 4) & 0x0F;
+        const int y = xy & 0x0F;
+        const int width = ((wh >> 4) & 0x0F) + 1;
+        const int height = (wh & 0x0F) + 1;
+        subrects.append(HextileSubrect{QRect(x, y, width, height), color});
+    }
+    return subrects;
+}
+
+}