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.

docs/PROTOCOL.md

@@ -181,22 +181,6 @@ Fichero-branded printers:
 - Fichero 4575 -> DP_D1H (Lujiang)
 - Fichero 4437 -> DP_L81H (Lujiang)
 
-## Print sequence
-
-The exact command sequence used by the official Fichero app, extracted from the decompiled APK and verified against hardware:
-
-```
-1. 10 FF 10 00 nn              Set density (0-2)
-2. 10 FF 84 00                  Set paper type (gap label)
-3. 00 x12                       Wake up (12 null bytes)
-4. 10 FF FE 01                  Enable printer (AiYin)
-5. 1D 76 30 00 0C 00 yL yH     Raster image (ESC/POS GS v 0)
-   [pixel data...]              96px wide, 1-bit, MSB first
-6. 1D 0C                        Feed to next label
-7. 10 FF FE 45                  Stop print job (AiYin)
-                                 wait for 0xAA or "OK"
-```
-
 ## How this was reverse-engineered
 
 1. BLE enumeration with bleak to find services and characteristics