Add VNC ZRLE decoding

Implements RFC 6143 SS7.7.6: a ZRLE rectangle is a 4-byte compressed
length followed by that many zlib-compressed bytes, decompressing to
64x64 tiles each using one of five subencodings (Raw, Solid, packed
palette, Plain RLE, Palette RLE). The zlib stream persists for the
whole connection rather than being reset per-rectangle or per-update,
so VncSessionBackend now owns a lazily-initialized, persistent
z_stream torn down only in resetProtocolState() on a fresh
connect/reconnect.

Since the entire rectangle's compressed data decompresses into memory
in one shot, tile parsing is a plain synchronous loop rather than
needing its own RfbState values -- only the compressed-length and
compressed-data reads are actual protocol states. Tile decoding (the
five subencodings, including the continuation-byte run-length
encoding shared by two of them) lives in vnc_pixel_codecs.h/.cpp
alongside the Hextile decoder, unit-tested with 6 new tests covering
each subencoding plus a persistence test that splits one continuous
deflate stream across two separate FramebufferUpdate messages -- it
only decodes correctly if the connection's inflate stream is retained
between them.

Adds a top-level find_package(ZLIB REQUIRED) + ZLIB::ZLIB link
(previously only pulled in transitively via vendored FreeRDP's own
smartcard-emulation feature, which happened to have it enabled but
shouldn't be relied on for that).

Live-verified against the TightVNC test server that nothing regressed
(connect, cursor, clipboard); that server consistently sends Raw for
actual framebuffer content regardless of announced encodings, so
Hextile/ZRLE's live decode path isn't independently confirmed against
a real server -- the unit tests are the primary evidence here.

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

CMakeLists.txt

@@ -14,6 +14,7 @@ include(GNUInstallDirs)
 
 find_package(Qt6 6.2 REQUIRED COMPONENTS Widgets Sql Network)
 find_package(OpenSSL REQUIRED)
+find_package(ZLIB REQUIRED)
 
 qt_standard_project_setup()
 
@@ -155,6 +156,7 @@ add_executable(orbithub WIN32 MACOSX_BUNDLE ${ORBITHUB_SOURCES})
 target_link_libraries(orbithub PRIVATE Qt6::Widgets Qt6::Sql Qt6::Network)
 target_link_libraries(orbithub PRIVATE KodoTerm::KodoTerm)
 target_link_libraries(orbithub PRIVATE OpenSSL::Crypto)
+target_link_libraries(orbithub PRIVATE ZLIB::ZLIB)
 target_compile_definitions(orbithub PRIVATE ORBITHUB_VERSION_STRING="${PROJECT_VERSION}")
 if(TARGET freerdp AND TARGET winpr)
     target_compile_definitions(orbithub PRIVATE ORBITHUB_HAS_FREERDP)