Split hardware-only deps into server/requirements-hw.txt

pyserial and spidev are Linux/Pi-only (spidev requires the kernel SPI
driver and won't install on macOS/Windows). Dev machines use
requirements.txt; the Pi uses requirements-hw.txt which pulls in the
base file via -r.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SH2D4LRuCnhLSzoYerAfJX

server/requirements-hw.txt

@@ -0,0 +1,3 @@
+-r requirements.txt
+pyserial
+spidev

server/requirements.txt

@@ -1,7 +1,5 @@
 numpy
 websockets==10.4
 dpath
-pyserial
-spidev
 # web/ (the browser client) needs none of these - it's static HTML/JS
 # served straight from the stdlib's http.server, no new dependency.