Circuit ↔ firmware cooperation

The circuit owns which MCU pad sits on which net; firmware-agent owns which
peripheral drives each pad. The two sides meet through JSON files in the
shared workspace (ADR-0023) and never import each other's code.

Mark the MCU in the brief

Set "mcu": true on the MCU part in the design brief. Every part with
mcu: true is exported; a design without one cannot be exported.

Export connectivity for firmware

circuit_firmware_export (CLI circuit firmware-export --brief B --netlist N --out DIR)
writes <design>.firmware.json (artifact_kind: circuit_firmware_connectivity):
per MCU ref, every pin with its package pin number, KiCad pinfunction
name (when a kicad-cli netlist is given), net, signal class and net
voltage, plus the sha256 of the brief and netlist it was derived from.
Pass the netlist whenever one exists: pad names such as GPIO4 come from
the KiCad pin functions; without it only package pins and nets are known.
Re-export after any brief or schematic change — firmware treats a hash
mismatch as stale.

Confirm the firmware pin map

firmware-agent writes <name>.fw-pinmap.json (artifact_kind: firmware_pinmap).
circuit_firmware_check (CLI circuit firmware-check --brief B --pinmap P [--netlist N] [--out F])
rebuilds the connectivity and checks every entry. It fails when:

• the MCU ref is not an mcu: true part, or the pin map is malformed;
• a pad (by name, alias or package pin) is not on an MCU pin;
• the pad's net differs from the pin map's net;
• a signal is assigned to a power or ground pin;
• the net voltage exceeds the MCU's max_io_v;
• an MCU pin on a signal net is left unassigned by firmware;
• the firmware supply net does not reach the MCU.

The check always re-derives connectivity from the current brief and
netlist, so it judges the pin map against the circuit as it is now; the
report records the brief, netlist, pin map and firmware contract hashes.

A pass from this check and a pass of firmware's fw.netlist_match
together confirm the pin map. On a mismatch, decide which side changes:
fix the brief/schematic and re-export, or answer the firmware
fw_request so firmware-agent reassigns the pad. Never edit the firmware
contract or its generated files from the circuit side.