P1-R3: Initial project infrastructure

- Project directory structure: docs/, src/, build/, recipes/, scripts/, wiki/
- README.md with project overview and status
- .editorconfig for consistent code style
- .gitignore for Python, C/C++, kernel, and build artifacts
- docs/research/: system research report + base OS decision
- docs/audio-stack-config.md: audio stack configuration notes
- config/: JACK, ALSA, CPU performance configs
- Preserved research from P1-R1 and P1-R2 tasks

config/99-audio.conf

@@ -0,0 +1,6 @@
+# Realtime audio group limits
+# Members of the 'audio' group get realtime scheduling and unlimited memory lock
+@audio   -   rtprio      99
+@audio   -   memlock     unlimited
+@audio   -   nice       -20
+@audio   -   priority    99

config/alsa-usb.conf

@@ -0,0 +1,7 @@
+# USB audio buffer tuning
+# Low-latency USB audio configuration
+
+# nrpacks=1: disable implicit feedback sync buffering
+# Reduces USB transfer latency at cost of slightly higher CPU
+options snd-usb-audio nrpacks=1
+

config/cmdline.txt

@@ -0,0 +1 @@
+console=serial0,115200 console=tty1 root=PARTUUID=xxxxxxxx-02 rootfstype=ext4 fsck.repair=yes rootwait isolcpus=1-3 nohz_full=1-3 rcu_nocbs=1-3 threadirqs irqaffinity=0 quiet

config/cpu-performance.service

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+[Unit]
+Description=Set CPU governor to performance
+After=multi-user.target
+
+[Service]
+Type=oneshot
+ExecStart=/bin/sh -c 'echo performance | tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor'
+RemainAfterExit=yes
+
+[Install]
+WantedBy=multi-user.target

config/jackd.service

@@ -0,0 +1,43 @@
+[Unit]
+Description=JACK2 Audio Server
+After=sound.target network.target
+Wants=sound.target
+
+[Service]
+Type=simple
+User=pi
+Group=audio
+Environment=JACK_NO_AUDIO_RESERVATION=1
+Environment=JACK_PROMISCUOUS_SERVER=1
+
+# Real-time scheduling
+LimitRTPRIO=99
+LimitMEMLOCK=infinity
+LimitNICE=-20
+
+# CPU affinity — pin JACK server to audio cores 1-3
+CPUAffinity=1-3
+
+# I/O scheduling — realtime class
+IOSchedulingClass=realtime
+IOSchedulingPriority=0
+
+# JACK daemon command
+ExecStart=/usr/bin/jackd \
+    -P 70 \
+    -t 2000 \
+    -d alsa \
+    -d hw:USB \
+    -r 48000 \
+    -p 128 \
+    -n 3 \
+    -M \
+    -X seq \
+    -S
+
+# Restart on crash (but not on clean shutdown)
+Restart=on-failure
+RestartSec=2
+
+[Install]
+WantedBy=multi-user.target

config/jackdrc

@@ -0,0 +1,2 @@
+# /etc/jackdrc — system-wide JACK2 default settings
+/usr/bin/jackd -P 70 -t 2000 -d alsa -d hw:USB -r 48000 -p 128 -n 3 -M -X seq

config/rtirq

@@ -0,0 +1,16 @@
+# /etc/default/rtirq — realtime IRQ thread priority tuning
+
+# List of IRQ thread names to boost (space-separated regex patterns)
+RTIRQ_NAME_LIST="xhci_hcd snd usb"
+
+# Highest priority for listed threads
+RTIRQ_PRIO_HIGH=95
+
+# Decrement priority per subsequent match
+RTIRQ_PRIO_DECR=1
+
+# Lowest priority for unlisted threads
+RTIRQ_PRIO_LOW=50
+
+# Reset IRQ thread priorities before applying (0=no, 1=yes)
+RTIRQ_RESET_IRQ=0