Eliza e1 demo board

The board target is a contract artifact for first-article bring-up planning, not a manufacturable PCB yet.

Minimal demo board contents:

• Placeholder QFN64 chip symbol and footprint.
• 3.3 V IO rail.
• 1.8 V core rail.
• External oscillator.
• Reset button.
• MCU/debug header driving the demo MMIO interface.
• 8 LEDs on GPIO outputs.
• IRQ test points.
• JTAG/debug header reserved.

The board must not be released for fabrication until the package, padframe, pinout, ESD, and rail-current assumptions are replaced with foundry/package-specific data.

Physical TH1520 reference-board procurement and validation are tracked in docs/board/th1520-procurement-test-plan.md.