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/gerber/pi-multifx-hat.drl

@@ -0,0 +1,64 @@
+M48
+; DRILL file {KiCad 9.0.8+dfsg-1} date 2026-06-09T19:00:51-0400
+; FORMAT={-:-/ absolute / metric / decimal}
+; #@! TF.CreationDate,2026-06-09T19:00:51-04:00
+; #@! TF.GenerationSoftware,Kicad,Pcbnew,9.0.8+dfsg-1
+; #@! TF.FileFunction,MixedPlating,1,2
+FMAT,2
+METRIC
+; #@! TA.AperFunction,Plated,PTH,ComponentDrill
+T1C1.000
+; #@! TA.AperFunction,Plated,PTH,ComponentDrill
+T2C1.500
+%
+G90
+G05
+T1
+X8.37Y-3.73
+X8.37Y-6.27
+X10.91Y-3.73
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+X13.45Y-3.73
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+X26.15Y-3.73
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+X38.85Y-3.73
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+X41.39Y-3.73
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+X43.93Y-3.73
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+X46.47Y-3.73
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+X49.01Y-3.73
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+X54.09Y-3.73
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+X56.63Y-3.73
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+T2
+X2.0Y-28.0
+X5.0Y-32.0
+X8.0Y-28.0
+X57.0Y-28.0
+X60.0Y-32.0
+X63.0Y-28.0
+M30