Add logemit event callbacks and kanta.tty terminal formatting

All change-related output (transactions, bootstrap, migrations) is now
described by a mutable LogEvent carrying full state plus the preferred
logger and level, and dispatched through emit_event. @kanta.logemit
callbacks receive the event and decide what is logged where: falsy return
marks it handled, truthy passes it (possibly modified) down the chain,
with default_emit - Kanta's own formatting, now just another emitter - as
the fallback. Pretty header and diff lines are lazy event properties.

New kanta.tty module: Line builder (call to append content, .colorname
arms a palette color for the next call with automatic folded reset,
width/align padding), a mutable Colors palette storing bare SGR params
(0 clears, sequential last-wins stacking), and strip_ansi/displaywidth/pad
helpers that count wide chars and emoji correctly.

tests/test_format_diff.py

@@ -1,4 +1,8 @@
-from kanta.logging import _ADD, _DELETE, format_diff
+from kanta.logging import format_diff
+from kanta.tty import ESC, colors
+
+_ADD = f"{ESC}{colors.add}m"
+_DELETE = f"{ESC}{colors.delete}m"
 
 
 def test_add():