fix: remove After=synth-diagnostic from pipeline services

Root cause: synth-diagnostic uses systemctl start to start pipeline
services, but those services had After=synth-diagnostic — causing
deadlock (they waited for synth-diagnostic to exit, which was waiting
for them).

Fix: pipeline services no longer list synth-diagnostic in After=.
synth-diagnostic has Before= them for boot ordering, and starts them
directly via systemctl start in run-tests.sh.

Also fixed prometheus-exporter: had After=synth-diagnostic backwards
— should start before synth-diagnostic (synth-diagnostic has
After=prometheus-exporter).

system/led-matrix-viz.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=LED Matrix MIDI Visualization
-After=arduino-router.service synth-diagnostic.service
+After=arduino-router.service
 
 [Service]
 User=arduino

system/midi-bridge.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=MIDI rawmidi → Pd UDP bridge (auto-detects gadget/host)
-After=multi-user.target synth-diagnostic.service
+After=multi-user.target
 
 [Service]
 Type=simple

system/pd-synth-onboard.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Pure Data Synth (Onboard PMIC Audio)
-After=multi-user.target synth-diagnostic.service
+After=multi-user.target
 Wants=midi-bridge.service
 
 [Service]

system/prometheus-exporter.service

@@ -1,6 +1,6 @@
 [Unit]
 Description=Prometheus metrics exporter for synth pipeline
-After=multi-user.target synth-diagnostic.service
+After=multi-user.target
 
 [Service]
 Type=simple