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.

README.md

@@ -66,7 +66,7 @@ Requires Web Bluetooth, so Chrome/Edge/Opera only. Firefox and Safari don't supp
 Requires Python 3.10+ and uv. Turn on the printer and run:
 
 ```
-uv run printer.py info
+uv run fichero info
 ```
 
 This auto-discovers the printer via BLE scan. To skip scanning on subsequent runs, find your printer's address from the scan output and save it:
@@ -78,45 +78,68 @@ export FICHERO_ADDR=AA:BB:CC:DD:EE:FF
 You can also pass it per-command:
 
 ```
-uv run printer.py --address AA:BB:CC:DD:EE:FF info
+uv run fichero --address AA:BB:CC:DD:EE:FF info
 ```
 
 ## CLI Usage
 
 ```
-uv run printer.py --help
+uv run fichero --help
 ```
 
 ### Printing
 
 ```
-uv run printer.py text "Hello World"
-uv run printer.py text "Fragile" --density 2 --copies 3
-uv run printer.py image label.png
-uv run printer.py image label.png --density 1 --copies 2
+uv run fichero text "Hello World"
+uv run fichero text "Fragile" --density 2 --copies 3
+uv run fichero text "Big Label" --font-size 40 --label-height 180
+uv run fichero image label.png
+uv run fichero image label.png --density 1 --copies 2
 ```
 
 Density: 0=light, 1=medium (default), 2=thick.
 
+Text labels accept `--font-size` (default 24) and `--label-height` in pixels (default 240).
+
 ### Device info
 
 ```
-uv run printer.py info
-uv run printer.py status
+uv run fichero info
+uv run fichero status
 ```
 
 ### Settings
 
 ```
-uv run printer.py set density 2
-uv run printer.py set shutdown 30
-uv run printer.py set paper gap
+uv run fichero set density 2
+uv run fichero set shutdown 30
+uv run fichero set paper gap
 ```
 
 - `density` - how dark the print is. 0 is faint, 1 is normal, 2 is the darkest. Higher density uses more battery and can smudge on some label stock.
-- `shutdown` - how many minutes the printer waits before turning itself off when idle. Set it higher if you're tired of turning it back on between prints.
+- `shutdown` - how many minutes the printer waits before turning itself off when idle (1-480). Set it higher if you're tired of turning it back on between prints.
 - `paper` - what kind of label stock you're using. `gap` is the default, for labels with spacing between them (the printer detects the gap to know where to stop). `black` is for rolls with a black mark between labels. `continuous` is for receipt-style rolls with no markings.
 
+## Library Usage
+
+```python
+import asyncio
+from fichero import connect, PrinterNotFound
+
+async def main():
+    async with connect() as pc:
+        info = await pc.get_info()
+        print(info)
+
+asyncio.run(main())
+```
+
+The package exports `PrinterClient`, `connect`, `PrinterError`, `PrinterNotFound`, `PrinterTimeout`, `PrinterNotReady`, and `PrinterStatus`.
+
+## TODO
+
+- [ ] Emoji support in text labels. The default Pillow font has no emoji glyphs, so they render as squares. Needs two-pass rendering: split text into emoji/non-emoji segments, render emoji with Apple Color Emoji (macOS) or Noto Color Emoji (Linux) using `embedded_color=True`, then composite onto the label.
+
 ## Protocol and reverse engineering
 
 See [docs/PROTOCOL.md](docs/PROTOCOL.md) for the full command reference, print sequence, and how this was reverse-engineered.