Replace log dict toggles with logdiff kwarg and kanta.transaction.diff logger

Per-transaction logdiff=False skips building and printing the diff body,
logging only the header. Globally, configure_logging(diff=False) disables
the kanta.transaction.diff child logger, which now carries all diff lines,
so applications can route or silence diffs separately from headers.

demo/main.py

@@ -1,15 +1,5 @@
-"""Kanta feature demo.
-
-Run from the project root:  python demo/main.py
-
-Demonstrates bootstrap, colored transaction diffs, logfmt value formatting,
-logging toggles, rollback, migrations, and a custom clock.
-The database is recreated with fixed timestamps on every run; everything else
-lives in this file.
-"""
-
+#!/usr/bin/env -S uv run
 import asyncio
-import logging
 import sys
 from datetime import UTC, datetime, timedelta
 from pathlib import Path
@@ -38,34 +28,24 @@ kanta_v0 = Kanta(filename, Data())
 @kanta_v0.bootstrap
 def bootstrap(data: Data) -> None:
     """Create the initial admin user."""
-    data.users["u1"] = {"name": "Alice", "role": "admin"}
+    data.users["userid001"] = {"name": "Alice", "role": "admin"}
 
 
-# Redefinition to simulate new version with counter renamed to total
+# Redefinition to simulate new version
 class Data(msgspec.Struct):
     users: dict[str, dict] = {}
-    total: int = 0
+    total: int = 0  # Replaces old counter field
+    lang: str = "en"  # New field
 
 
 def migrate_v1(d: dict) -> None:
     """Rename counter to total"""
-    d["total"] = d.pop("counter")
+    d["total"] = d["counter"]
 
 
 kanta_v1 = Kanta(filename, Data(), migrations=sys.modules[__name__])
 
 
-_now = datetime(2027, 1, 1, tzinfo=UTC)
-
-
-@kanta_v0.clock
-@kanta_v1.clock
-def fake_clock() -> datetime:
-    global _now
-    _now += timedelta(hours=1)
-    return _now
-
-
 @kanta_v0.logfmt
 @kanta_v1.logfmt
 def resolve_user(
@@ -82,61 +62,61 @@ def resolve_user(
     return None
 
 
-def section(title: str) -> None:
-    print(f"\n# {title}", flush=True)
-
-
 async def main() -> None:
     filename.unlink(missing_ok=True)
 
-    section("Database creation with v0 schema and basic access")
+    print("# Database creation with v0 schema and basic ops, pretty logs", flush=True)
     # Open and close automatically; you can also `await kanta.open()` instead
     async with kanta_v0 as kanta:
-        with kanta.transaction(action="create", user="u2") as data:
-            data.users["u2"] = {"name": "Bob", "role": "user"}
+        with kanta.transaction(action="create", user="userid001") as data:
+            data.users["userid002"] = {"name": "Bob", "role": "user"}
 
-        with kanta.transaction(action="update", user="u1") as data:
-            data.users["u2"]["role"] = "editor"
+        with kanta.transaction(action="update", user="userid001") as data:
+            data.users["userid002"]["role"] = "editor"
             data.counter = 1
 
-        with kanta.transaction(action="delete", user="u1") as data:
-            del data.users["u2"]
+        with kanta.transaction(action="delete", user="userid002") as data:
+            del data.users["userid001"]
 
         # Display-only extra string, appended after the action.
-        with kanta.transaction(action="export", user="u1", extra=filename.name) as data:
+        with kanta.transaction(
+            action="export", user="userid002", extra="extra info"
+        ) as data:
             data.counter = 2
 
-        # Compact logging, diff only: a system fix stamped by the clock, but the
-        # modification time (m) is not updated.
-        with kanta.transaction(
-            action="repair", mtime=False, log={"header": False, "diff": True}
-        ) as data:
-            data.users["u3"] = {"name": "Carol", "role": "user"}
-
-        # A failing transaction rolls back and logs a warning.
         try:
-            with kanta.transaction(action="reset", user="u1") as data:
+            with kanta.transaction(action="reset") as data:
                 data.counter = 99
                 raise ValueError("simulated failure")
         except ValueError:
-            pass
+            print(f"# Reading does not need transaction: {data.counter=}", flush=True)
 
-        # Compact logging, header only.
-        with kanta.transaction(
-            action="import", user="u1", log={"header": True, "diff": False}
-        ) as data:
+        with kanta.transaction(action="import", logdiff=False) as data:
             data.counter = 3
 
-    section("A later version of our application with new data model and migrations")
+    print(
+        "\n# A later version of our application with new data model and migrations",
+        flush=True,
+    )
     async with kanta_v1 as kanta:
-        with kanta.transaction(action="update", user="u1", extra=filename.name) as data:
+        with kanta.transaction(
+            action="update", user="userid002", extra=filename.name
+        ) as data:
             data.total = 4
 
-        with kanta.transaction(action="create", user="u3", extra="Dave (u4)") as data:
-            data.users["u4"] = {"name": "Dave", "role": "user"}
+
+# Fake clock for deterministic timestamps
+_now = datetime(2027, 1, 1, tzinfo=UTC)
+
+
+@kanta_v0.clock
+@kanta_v1.clock
+def fake_clock() -> datetime:
+    global _now
+    _now += timedelta(hours=1)
+    return _now
 
 
 if __name__ == "__main__":
     configure_logging()
-    logging.getLogger("kanta").setLevel(logging.DEBUG)  # show migration diffs
     asyncio.run(main())