Rewrite the driver in Rust; move the testbench tools out

The repository is now only the HPCS 6500 driver: a Rust library (serial
transport, every protocol command, block parsing, USBPcap captures, a
simulated instrument) and the hpcs6500 CLI with the flags of the former
hpcs6500.py, plus dump/capture for reverse engineering.

The lamp station, lamp export, TM-30, label printer and LED calibration
move to B4L/lamp-testbench. Golden tests pin the Rust output to the Python
driver's: parsed values bit-exact, report text, CSV and number formatting.
The Python driver is at tag python-final.

src/error.rs

@@ -0,0 +1,52 @@
+use std::fmt::Write;
+
+/// Everything that can go wrong talking to the instrument.
+#[derive(Debug, thiserror::Error)]
+pub enum Error {
+    #[error("serial port: {0}")]
+    Serial(#[from] serialport::Error),
+    #[error("I/O: {0}")]
+    Io(#[from] std::io::Error),
+    /// A write command was not acknowledged with `8C <cmd>`.
+    #[error("no acknowledge for command 0x{cmd:02X} (got {got})")]
+    NoAck { cmd: u8, got: String },
+    /// A read command answered with an unexpected header.
+    #[error("bad response to command 0x{cmd:02X} (got {got})")]
+    BadResponse { cmd: u8, got: String },
+    /// A data block ended early.
+    #[error("{what} short ({got}/{want} bytes)")]
+    Short {
+        what: &'static str,
+        got: usize,
+        want: usize,
+    },
+    #[error("timeout waiting for measurement data")]
+    Timeout,
+    /// The cancel flag (Ctrl+C) was raised during a wait.
+    #[error("cancelled")]
+    Cancelled,
+    #[error("HPCS 6500 not found (no serial port with VID 0483 / PID 5741)")]
+    NotFound,
+    #[error("pcap: {0}")]
+    Pcap(String),
+}
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+/// `8c 13 0f` style hex for error messages, `(nothing)` when empty.
+pub(crate) fn hex(b: &[u8]) -> String {
+    if b.is_empty() {
+        return "nothing".into();
+    }
+    let mut s = String::new();
+    for (i, x) in b.iter().take(24).enumerate() {
+        if i > 0 {
+            s.push(' ');
+        }
+        let _ = write!(s, "{x:02x}");
+    }
+    if b.len() > 24 {
+        let _ = write!(s, " ... ({} bytes)", b.len());
+    }
+    s
+}