Add trade ledger debugging exercise

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

logger.py

@@ -0,0 +1,68 @@
+import sys
+import time
+
+LEVELS = ("DEBUG", "INFO", "WARNING", "ERROR")
+
+
+class Record:
+    __slots__ = ("ts", "level", "msg", "context")
+
+    def __init__(self, ts, level, msg, context):
+        self.ts = ts
+        self.level = level
+        self.msg = msg
+        self.context = context
+
+    def render(self):
+        ctx = " ".join(f"{k}={v}" for k, v in sorted(self.context.items()))
+        return f"{self.ts:.6f} {self.level:<7} {self.msg} {ctx}".rstrip()
+
+
+class BufferedLogger:
+    def __init__(self, name, level="INFO", stream=None, capacity=64):
+        self.name = name
+        self.stream = stream if stream is not None else sys.stderr
+        self.capacity = capacity
+        self._context = {}
+        self._buffer = []
+        self.set_level(level)
+
+    def set_level(self, level):
+        level = level.upper()
+        if level not in LEVELS:
+            raise ValueError(f"unknown level {level!r}")
+        self.level = level
+
+    def push(self, **fields):
+        self._context.update(fields)
+
+    def clear(self):
+        self._context.clear()
+
+    def _enabled(self, level):
+        return level >= self.level
+
+    def _log(self, level, msg):
+        if not self._enabled(level):
+            return
+        self._buffer.append(Record(time.time(), level, msg, self._context))
+        if len(self._buffer) >= self.capacity:
+            self.flush()
+
+    def debug(self, msg):
+        self._log("DEBUG", msg)
+
+    def info(self, msg):
+        self._log("INFO", msg)
+
+    def warning(self, msg):
+        self._log("WARNING", msg)
+
+    def error(self, msg):
+        self._log("ERROR", msg)
+
+    def flush(self):
+        for rec in self._buffer:
+            self.stream.write(f"[{self.name}] {rec.render()}\n")
+        self.stream.flush()
+        self._buffer.clear()