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.h

@@ -0,0 +1,65 @@
+#ifndef ORBITHUB_VNC_PIXEL_CODECS_H
+#define ORBITHUB_VNC_PIXEL_CODECS_H
+
+#include <QByteArray>
+#include <QRect>
+#include <QRgb>
+#include <QVector>
+
+// Pure, state-free decode helpers for VNC/RFB pixel encodings, kept out of
+// VncSessionBackend so the wire-sequencing (when to read what off the
+// socket) and wire-decoding (how to interpret already-buffered bytes) stay
+// separate and the latter is unit-testable without a live connection.
+namespace VncPixelCodecs {
+
+// Converts one pixel's worth of raw bytes as sent under
+// VncSessionBackend's negotiated SetPixelFormat (32bpp, little-endian,
+// byte order B,G,R,pad) into a QRgb.
+QRgb rgbFromPixelBytes(const uchar* bytes);
+
+// A single filled rectangle, in tile-local coordinates (0,0 = the tile's
+// own top-left corner, not the enclosing FramebufferUpdate rectangle's).
+struct HextileSubrect {
+    QRect rect;
+    QRgb color = 0;
+};
+
+// RFC 6143 SS7.7.4 Hextile tile subencoding byte flags.
+namespace HextileFlags {
+constexpr quint8 kRaw = 0x01;
+constexpr quint8 kBackgroundSpecified = 0x02;
+constexpr quint8 kForegroundSpecified = 0x04;
+constexpr quint8 kAnySubrects = 0x08;
+constexpr quint8 kSubrectsColoured = 0x10;
+}
+
+// Byte length of a Hextile tile's "fixed" metadata -- the optional
+// background/foreground color updates plus the optional subrect count --
+// derivable from the subencoding byte alone, before any of those bytes are
+// available. Only meaningful when HextileFlags::kRaw is *not* set (a Raw
+// tile has no metadata at all, just tileWidth*tileHeight raw pixels).
+int hextileFixedMetaByteCount(quint8 subencoding);
+
+// Parses the `hextileFixedMetaByteCount(subencoding)` bytes described
+// above. Updates *background/*foreground in place only when the
+// corresponding flag is set in `subencoding` -- callers should persist
+// their previous values across tiles in the same rectangle and pass them
+// in here unchanged when a color isn't re-specified, since RFC 6143 has
+// each tile inherit the last-specified colors. Returns the subrect count
+// (0 if HextileFlags::kAnySubrects isn't set).
+int decodeHextileFixedMeta(quint8 subencoding, const QByteArray& data, QRgb* background,
+                           QRgb* foreground);
+
+// Byte length of `subrectCount` subrects' worth of data, given whether
+// they're individually colored (HextileFlags::kSubrectsColoured).
+int hextileSubrectByteCount(bool coloured, int subrectCount);
+
+// Parses `subrectCount` subrects (xy + wh bytes, plus a per-subrect color
+// when `coloured`) out of `data`, substituting `foreground` in for any
+// that aren't individually colored.
+QVector<HextileSubrect> decodeHextileSubrects(bool coloured, int subrectCount,
+                                              const QByteArray& data, QRgb foreground);
+
+}
+
+#endif