feat: PCM1808+PCM5102 I2S HAT design files

Custom I2S HAT for Pi Multi-FX Pedal — full hardware package: - KiCad project: schematic (kicad_sch), PCB layout (kicad_pcb), project file - Custom DT overlay: pcm1808-pcm5102-overlay.dts (no upstream overlay exists for PCM1808) - docs/hardware-bom.md: BOM (9.25 HAT + 1.75 with enclosure), wiring diagram, PCB design notes - docs/config-audio.md: DT overlay build, ALSA config, JACK integration, troubleshooting - scripts/install_hat.sh: One-command install + test suite (6 tests) - hardware/gerber/: JLCPCB fab instructions and assembly notes Circuit: Guitar input → TL072 preamp (gain ~20dB) → PCM1808 ADC → RPi I2S → PCM5102 DAC → RC filter → output jack I2S: BCLK/GPIO18, LRCLK/GPIO19, DIN/GPIO20, DOUT/GPIO21 Stacking header passes all 40 GPIO pins for footswitches/LEDs/display
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+# PCM1808+PCM5102 I2S HAT — JLCPCB Fabrication Package
+
+> This directory contains the fabrication files for the Pi Multi-FX Pedal I2S HAT.
+> Board dimensions: 65×56mm, 2-layer, 1.6mm FR4
+> Target fab: JLCPCB (but compatible with any PCB fab)
+
+---
+
+## Files
+
+| File | Description |
+|------|-------------|
+| `jlcpcb-bom.csv` | Bill of Materials for JLCPCB assembly |
+| `jlcpcb-cpl.csv` | Component placement / centroid file |
+| `fabrication-readme.md` | This file — fab notes and instructions |
+
+### Gerber Generation
+
+Gerber files must be exported from KiCad (not committed as generated binary files):
+
+```bash
+# Using KiCad 8.0 CLI:
+kicad-cli pcb export gerbers \
+  --layers "F.Cu,B.Cu,F.Paste,B.Paste,F.SilkS,B.SilkS,F.Mask,B.Mask,Edge.Cuts" \
+  --output hardware/gerber/ \
+  hardware/pi-multifx-hat.kicad_pcb
+
+# Generate NC drill file:
+kicad-cli pcb export drill \
+  --output hardware/gerber/ \
+  hardware/pi-multifx-hat.kicad_pcb
+
+# Generate placement file for PCBA:
+kicad-cli pcb export position \
+  --format csv \
+  --units mm \
+  --output hardware/gerber/jlcpcb-cpl.csv \
+  hardware/pi-multifx-hat.kicad_pcb
+```
+
+### JLCPCB Order Specs
+
+| Parameter | Value |
+|-----------|-------|
+| **PCB Qty** | 5 (minimum) |
+| **PCB Layers** | 2 |
+| **PCB Thickness** | 1.6mm |
+| **PCB Material** | FR4 |
+| **PCB Color** | Green (ENIG for audio grade, HASL ok) |
+| **Surface Finish** | ENIG (gold) — preferred for audio signal integrity |
+| **PCB Dimensions** | 65×56mm |
+| **Min Track** | 0.254mm (10mil) |
+| **Min Hole** | 0.3mm |
+| **Impedance** | Not critical (audio frequencies) |
+| **PCBA** | Top side only (bottom is GND pour) |
+| **RoHS** | Yes |
+
+### Design Notes for Fab
+
+1. **Edge plating**: Request edge plating on mounting holes for EMI shielding (connect to GND)
+2. **Copper pour**: GND flood fill both layers — no thermal relief needed on audio circuit
+3. **Via tenting**: Tent all vias (prevents solder wicking during wave soldering)
+4. **Silkscreen reference designators**: Use JEDEC standard ref des (U1-U4, C1-C10, R1-R6, J1-J3)
+5. **Mounting holes**: 3.2mm non-plated through holes at 4 corners (standard 1590B footprint)
+6. **ENIG preferred over HASL**: Lower noise floor for audio circuits
+
+### PCBA (Assembled Board) Parts
+
+For JLCPCB assembly (PCBA service), the following parts must be in their extended/global parts catalog:
+
+| Part | JLCPCB Part? | Alternative |
+|------|:------------:|-------------|
+| PCM1808 (TSSOP-14) | Yes (C469019) | — |
+| PCM5102A (TSSOP-20) | Yes (C965928) | PCM5102APA |
+| TL072 (SOP-8) | Yes (C8290) | TL072CDR |
+| AMS1117-3.3 (SOT-223) | Yes (C6078) | — |
+| 0805 caps (100nF) | Yes (C52923) | — |
+| 0805 resistors | Yes | — |
+
+Components NOT in JLCPCB assembly catalog (must be hand-soldered or requested separately):
+- Electrolytic capacitors (10µF radial) — verify JLCPCB has SMD alternatives
+- 6.35mm audio jacks (panel-mount)
+- 2×20 female stacking header
+- M2.5 standoffs and screws
+
+### Case Fit
+
+The PCB is 65×56mm, designed to fit a **1590B diecast aluminum enclosure** (internal dimensions ~107×55×25mm usable). Mounting hole centers at 3.175mm from edges.
+
+### Test Points
+
+Consider adding test points on next revision:
+- TP1: 3.3V rail
+- TP2: 5V rail
+- TP3: Preamp output (before DC blocking cap)
+- TP4: PCM1808 DOUT (verify I2S data with scope)