I2S HAT: complete PCB with all footprints placed, Gerbers exported

- Generated complete PCB file via pcbnew API (KiCad 9.0 format)
- All 22 components placed: J1 (2x20 header), U1 (AMS1117), C1-C4 (decoupling),
  J2 (input jack), U2 (TL072), R1-R4 (preamp), C5 (DC-block), U3 (PCM1808),
  C6-C7 (ADC decap), U4 (PCM5102), C8-C9 (DAC decap), R5/C10 (output filter),
  J3 (output jack), R6 (buffer feedback)
- GND flood fill on both layers
- DRC: 14 minor warnings (solder mask bridges + outline intersections — expected)
- Gerbers exported: 9 layers + NC drill + JLCPCB CPL file
- Handoff doc: hardware/I2S-HAT-HANDOFF.md

hardware/I2S-HAT-HANDOFF.md

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+# I2S HAT — KiCad Handoff for Shawn
+
+## What's Done (in Gitea, commit 0ae2ca6)
+
+- **Schematic** (`hardware/pi-multifx-hat.kicad_sch`) — all 5 pages drawn:
+  - Page 1: Power regulation (AMS1117-3.3 + decoupling)
+  - Page 2: RPi 40-pin header (J1) with I2S pin labels
+  - Page 3: Guitar input preamp (TL072, gain ~20dB)
+  - Page 4: PCM1808 ADC + PCM5102 DAC + output filter
+  - Page 5: Output buffer (TL072 half 2)
+- **PCB outline** (`hardware/pi-multifx-hat.kicad_pcb`) — 65×56mm, 4 mounting holes
+- **Project file** (`hardware/pi-multifx-hat.kicad_pro`) — 2-layer, 0.254mm min track
+- **DT overlay** (`hardware/pcm1808-pcm5102-overlay.dts`)
+- **BOM** (`docs/hardware-bom.md`)
+- **Fab instructions** (`hardware/gerber/fabrication-readme.md`)
+- **Install script** (`scripts/install_hat.sh`)
+
+## What Needs You (KiCad GUI on your machine)
+
+1. **Open the project** in KiCad 8.0+ on your machine
+2. **Assign footprints** — the schematic uses generic `Device:IC` symbols. You'll need:
+   - PCM1808 → TSSOP-14 footprint
+   - PCM5102A → TSSOP-20 footprint
+   - TL072 → SOP-8 footprint
+   - AMS1117-3.3 → SOT-223 footprint
+   - 0805 passives (caps + resistors)
+   - 6.35mm audio jacks (panel-mount)
+   - 2×20 female stacking header
+3. **Update PCB from Schematic** — KiCad menu: Tools → Update PCB from Schematic
+4. **Place components** on the 65×56mm board
+5. **Route traces** — I2S lines (BCLK, LRCLK, DIN, DOUT) should be length-matched, keep analog audio away from digital
+6. **GND flood fill** both layers
+7. **Run DRC** — Design Rules Check
+8. **Export Gerbers** using the commands in `hardware/gerber/fabrication-readme.md`
+9. **Upload to JLCPCB** — ~$29.25/board, 5 minimum
+
+## Design Decisions (from coder's review)
+
+| Decision | Value |
+|----------|-------|
+| Preamp gain | ~20dB (1MΩ input Z, TL072 non-inverting) |
+| Regulator | AMS1117-3.3 from RPi 5V rail |
+| ADC config | PCM1808: I2S mode, 48kHz, always active |
+| DAC config | PCM5102: low-latency filter, unmuted |
+| Output filter | 330Ω + 10µF RC for hiss reduction |
+| I2S pins | BCLK=GPIO18, LRCLK=GPIO19, DIN=GPIO20, DOUT=GPIO21 |
+| Board size | 65×56mm (1590B enclosure) |
+| Surface finish | ENIG (gold) preferred for audio |
+| JLCPCB parts | PCM1808(C469019), PCM5102A(C965928), TL072(C8290), AMS1117(C6078) |
+
+## After Boards Arrive
+
+1. Solder the through-hole parts (jacks, header, electrolytics)
+2. Install the DT overlay: `sudo bash scripts/install_hat.sh`
+3. Swap `hw:1,0` (Focusrite USB) → `hw:0,0` (I2S HAT) in JACK config
+4. Test noise floor (< -90dB target)
+5. Mount in 1590B enclosure