Take mouse button codes from curses, not hardcoded integers

[leancode]

Mouse mode (F8) only responded to left click. Right click and both scroll directions did nothing.

Cause

handle_mouse compared event.mouse_code against hardcoded integers:

if event.mouse_code == 1:            # left release
elif event.mouse_code == 4096:       # right release
elif event.mouse_code == 524288:     # wheel up
elif event.mouse_code == 134217728:  # wheel down

Those are NCURSES_MOUSE_VERSION 1 values, which allocated six bits per button. Version 2 packs them into five to make room for button 5, so every constant moved:

action	hardcoded	actual (ncurses 6.5)
left release	1	BUTTON1_RELEASED = 1	worked
right release	4096	BUTTON3_RELEASED = 1024	dead
wheel up	524288	BUTTON4_PRESSED = 65536	dead
wheel down	134217728	BUTTON5_PRESSED = 2097152	dead

Left click worked by coincidence — BUTTON1_RELEASED is 1 under both versions.

Fix

Match against the curses constants. BUTTON5 is only defined when ncurses is built with mouse version 2, so it falls back to 0 and wheel-down is simply unavailable there rather than mismatched.

Verification

Driving the real handle_mouse with the constants this ncurses reports:

action	before	after
left click	set_single_cursor	set_single_cursor
right click	unhandled	add_cursor
wheel up	unhandled	jump_up
wheel down	unhandled	jump_down

An unrecognised code still returns False. test.sh passes, flake8 clean.

Same class of bug as the hardcoded 5xx keycodes fixed in 0.3.0.