Add LAUNCH.md: startup sequence and why it fails without a reader

development is x86_64 with no RFID reader attached, so the reader path exits 1
by design. That looks like breakage when it is not, so document it.

- The expected output on this host, including why /dev/ttyS0 gives
  'Input/output error' (node exists, hardware does not answer) rather than
  'device not found' - the app distinguishes the two.
- The 5-stage startup as a diagram, with the code and exit code for each stage,
  and why stages 1-2 fail fast on purpose.
- How to exercise the reader path with no hardware: the PTY tests, plus a
  standalone snippet that writes a real RDM6300 frame. Both the snippet and the
  documented outputs were run verbatim, not written from memory.
- The pipe trap: head closes the pipe early and the shell then reports
  128+SIGPIPE, so redirect to a file when checking the real exit code.

README.md

@@ -5,6 +5,10 @@ print what happens: **card inserted** with the card data, then **card removed**.
 
 Built for a Raspberry Pi workstation, but it runs anywhere with a serial port.
 
+> Starting it on a machine with no reader attached? See **[LAUNCH.md](LAUNCH.md)**
+> — it explains the startup sequence, why it exits `1` with no hardware, and how
+> to exercise the reader path anyway.
+
 ---
 
 ## What it does