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_logemit.py

@@ -0,0 +1,157 @@
+import logging
+
+import pytest
+
+from kanta.logging import (
+    LogEvent,
+    bootstrap_logger,
+    configure_logging,
+    emit_event,
+    migration_logger,
+    transaction_logger,
+)
+from tests.support import Data, make_kanta
+
+
+@pytest.fixture(autouse=True)
+def _reset_kanta_loggers():
+    yield
+    for name in (
+        "kanta",
+        "kanta.transaction",
+        "kanta.transaction.diff",
+        "kanta.bootstrap",
+        "kanta.migration",
+    ):
+        logger = logging.getLogger(name)
+        logger.setLevel(logging.NOTSET)
+        logger.propagate = True
+        logger.disabled = False
+        logger.handlers.clear()
+
+
+def _change_event(**kwargs) -> LogEvent:
+    return LogEvent(kind="change", logger=transaction_logger, action="update", **kwargs)
+
+
+def test_emit_event_falsy_return_stops_chain(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+    calls = []
+
+    def first(ev):
+        calls.append("first")
+        return None
+
+    def second(ev):
+        calls.append("second")
+
+    emit_event(_change_event(), [first, second])
+    assert calls == ["first"]
+    assert capsys.readouterr().err == ""
+
+
+def test_emit_event_truthy_return_falls_back_to_default(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+    emit_event(_change_event(), [lambda ev: True])
+    assert "update" in capsys.readouterr().err
+
+
+def test_emit_event_mutation_reaches_later_handlers_and_default(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+    calls = []
+
+    def first(ev):
+        calls.append("first")
+        ev.extra = "tgt"
+        return True
+
+    def second(ev):
+        calls.append(("second", ev.extra))
+        return True
+
+    emit_event(_change_event(), [first, second])
+    assert calls == ["first", ("second", "tgt")]
+    assert "tgt" in capsys.readouterr().err
+
+
+def test_emit_event_handler_error_falls_back_to_default(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+
+    def boom(ev):
+        raise RuntimeError("broken")
+
+    emit_event(_change_event(), [boom])
+    assert "update" in capsys.readouterr().err
+
+
+def test_diff_lines_built_lazily(monkeypatch):
+    def _boom(*args, **kwargs):
+        raise AssertionError("format_diff should not be called")
+
+    monkeypatch.setattr("kanta.logging.format_diff", _boom)
+    ev = _change_event(diff={"counter": 1})
+    emit_event(ev, [lambda ev: None])  # handled without touching the diff
+    monkeypatch.undo()
+    assert len(ev.diff_lines) == 1
+    assert "counter" in ev.diff_lines[0]
+
+
+def test_default_emit_created_and_migrated(capsys):
+    logging.getLogger("kanta").handlers.clear()
+    configure_logging()
+    emit_event(LogEvent(kind="created", logger=bootstrap_logger, filename="x.kantadb"))
+    emit_event(
+        LogEvent(
+            kind="migrated",
+            logger=migration_logger,
+            filename="x.kantadb",
+            from_version=0,
+            to_version=1,
+            migrations=["migrate_v1 (rename)"],
+        )
+    )
+    err = capsys.readouterr().err
+    assert "Created x.kantadb" in err
+    assert "Migrated x.kantadb v0 -> v1: migrate_v1 (rename)" in err
+
+
+@pytest.mark.asyncio
+async def test_logemit_receives_transaction_events(tmp_path, format_config):
+    path = tmp_path / "test.db"
+    kanta = make_kanta(path, Data, format_config)
+    events = []
+    kanta.logemit(lambda ev: events.append(ev) or True)
+    await kanta.open()
+
+    with kanta.transaction(action="inc", user="u1", extra="x") as data:
+        data.counter = 1
+
+    await kanta.close()
+
+    change = events[-1]
+    assert change.kind == "change"
+    assert change.action == "inc"
+    assert change.user == "u1"
+    assert change.extra == "x"
+    assert change.diff == {"counter": 1}
+    assert change.logger.name == "kanta.transaction"
+
+
+def test_logemit_rejects_classes_and_async(tmp_path, format_config):
+    kanta = make_kanta(tmp_path / "test.db", Data, format_config)
+
+    class NotAFunction:
+        pass
+
+    with pytest.raises(TypeError):
+        kanta.logemit(NotAFunction)
+
+    async def ahandler(ev):
+        return None
+
+    with pytest.raises(TypeError):
+        kanta.logemit(ahandler)