Lestrix 0.4.0

• Rewritten from C to C++20: GLib is gone completely (standard library plus a small util layer), no GTK anywhere, smart pointers and std::function instead of manual frees and void* contexts. Terminal engine speed is unchanged (README has the 0.3.0 vs 0.4.0 table)
• Built-in fast cat: cat bigfile and producer | cat in a local tab are finished by the terminal reading the data itself (256 MB: 0.35 s instead of about 6.5 s). No helper binary, no PATH wrapper; toggle in View > Fast cat or --no-fastcat
• New --basic mode: one bare terminal with no menu bar, tabs, sidebar or stats; own desktop entry
• Coloured tabs now colour the tab background, with dark or light text for contrast
• Cache statistics: Ctrl+Shift+I (remappable in settings.ini [keys]), kill -USR1, or --cache-stats
• Lower resource use in all three modes: the cursor stops blinking after 10 s idle (0 idle wake-ups in Lite and Basic), helper threads start on demand, adaptive frame cap while output floods, faster colour output in the engine
• Fix: the pin button in the sidebar rail drew nothing during the rewrite; restored
• Removed: the lxcat helper (pipelines and files are handled by the terminal itself)