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>

tests/CMakeLists.txt

@@ -29,6 +29,7 @@ add_test(NAME test_ssh_session_backend COMMAND test_ssh_session_backend)
 add_executable(test_vnc_session_backend
     test_vnc_session_backend.cpp
     ${CMAKE_SOURCE_DIR}/src/vnc_session_backend.cpp
+    ${CMAKE_SOURCE_DIR}/src/vnc_pixel_codecs.cpp
     ${CMAKE_SOURCE_DIR}/src/session_backend.h
 )
 target_include_directories(test_vnc_session_backend PRIVATE ${CMAKE_SOURCE_DIR}/src)