Restructure into fichero/ package and fix review issues

Split monolithic printer.py into fichero/{printer,imaging,cli}.py.
Add asyncio.Lock to prevent notification buffer races, replace sys.exit
with PrinterNotFound/PrinterTimeout/PrinterNotReady exceptions, name all
magic sleep values as DELAY_* constants, add image_to_raster validation,
add --font-size and --label-height to text subcommand, add input
validation to set command, add type annotations to public API, fix
misleading paper type comment, remove duplicate PROTOCOL.md section,
update README for new CLI entry point and library usage.

fichero/__init__.py

@@ -0,0 +1,21 @@
+"""Fichero D11s thermal label printer - BLE interface."""
+
+from fichero.printer import (
+    PrinterClient,
+    PrinterError,
+    PrinterNotFound,
+    PrinterNotReady,
+    PrinterStatus,
+    PrinterTimeout,
+    connect,
+)
+
+__all__ = [
+    "PrinterClient",
+    "PrinterError",
+    "PrinterNotFound",
+    "PrinterNotReady",
+    "PrinterStatus",
+    "PrinterTimeout",
+    "connect",
+]