Keyboard input injection via BMP (0x72)

## Summary

Inject PETSCII text into the C64 keyboard buffer from Ghidra. Useful for typing BASIC commands, entering game passwords, or triggering text-based input handlers during a debugging session without switching focus to VICE.

Source: VICE BMP `Keyboard Feed` (0x72); confirmed in VICE source `src/monitor/monitor_binary.c`.

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## BMP protocol details

**Command:** `CMD_KEYBOARD_FEED = 0x72`

Request payload:
```
text_len (2 LE)
text (text_len bytes, PETSCII encoding)
```

Response: standard ACK. VICE inserts the bytes into the keyboard buffer as if typed.

**PETSCII encoding notes:**
- Uppercase letters: same as ASCII (`A`=0x41)
- Lowercase letters in PETSCII graphics mode: offset; in text mode same as ASCII
- Common control chars: `RETURN`=0x0D, `SPACE`=0x20, `HOME`=0x13, `CLR`=0x93, `RUN`=0x83
- For plain ASCII text injected as BASIC input, standard ASCII bytes work for printable chars

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## Implementation plan

### 1. `util.py`

```python
CMD_KEYBOARD_FEED = 0x72

def keyboard_feed(self, text: str):
"""
Inject text into the VICE keyboard buffer.
`text` should be ASCII; for RETURN append '\\r'.
"""
enc = text.encode('ascii')
payload = struct.pack('