feat: VoiletXterm - Qt5 terminal workspace for SSH, Telnet and serial

Terminal core (src/core, QtCore only, headless testable):
- VT100/xterm parser: CSI/OSC/DCS, SGR 256/truecolor, scroll regions,
  insert/delete, DEC special graphics, alternate screen
- cell grid with 5000 line scrollback, CJK wide chars, combining marks
- xterm 256 color palette

Terminal widget (src/term):
- QPainter renderer with run batching, wide char clipping, cursor blink
- mouse selection (char/word/line), auto copy, middle click paste
- full key encoding: arrows + DECCKM, F1-F12, modifiers, ctrl/alt
- IME support via inputMethodEvent

Transports (src/session):
- SSH through the system OpenSSH client on a forkpty pseudo terminal,
  so agent, ProxyJump, known_hosts and 2FA keep working
- Telnet with IAC negotiation (NAWS, ECHO, SGA)
- Serial via QSerialPort

UI (src/ui):
- tabbed main window, session dialog with saved profiles
- dark violet theme, CLI targets (user@host:port, telnet://, serial:)

Tests:
- tests/test_core: escape sequence and layout assertions
- tests/test_widget: offscreen render checks and key encoding

VoiletXterm.pro

@@ -0,0 +1,51 @@
+QT += core gui widgets network serialport
+
+TARGET = voiletxterm
+TEMPLATE = app
+CONFIG += c++17 warn_on
+CONFIG -= app_bundle
+
+DEFINES += QT_DEPRECATED_WARNINGS
+
+INCLUDEPATH += src
+
+SOURCES += \
+    src/main.cpp \
+    src/core/CharWidth.cpp \
+    src/core/Palette.cpp \
+    src/core/Screen.cpp \
+    src/core/VtParser.cpp \
+    src/term/TerminalTheme.cpp \
+    src/term/TerminalWidget.cpp \
+    src/session/PtyProcess.cpp \
+    src/session/SerialTransport.cpp \
+    src/session/SessionFactory.cpp \
+    src/session/SessionSpec.cpp \
+    src/session/SshTransport.cpp \
+    src/session/TelnetTransport.cpp \
+    src/ui/MainWindow.cpp \
+    src/ui/SessionDialog.cpp \
+    src/ui/SessionStore.cpp \
+    src/ui/Style.cpp
+
+HEADERS += \
+    src/core/Cell.h \
+    src/core/CharWidth.h \
+    src/core/Palette.h \
+    src/core/Screen.h \
+    src/core/VtParser.h \
+    src/term/TerminalTheme.h \
+    src/term/TerminalWidget.h \
+    src/session/PtyProcess.h \
+    src/session/SerialTransport.h \
+    src/session/SessionFactory.h \
+    src/session/SessionSpec.h \
+    src/session/SshTransport.h \
+    src/session/TelnetTransport.h \
+    src/session/Transport.h \
+    src/ui/MainWindow.h \
+    src/ui/SessionDialog.h \
+    src/ui/SessionStore.h \
+    src/ui/Style.h
+
+unix: LIBS += -lutil