Add MODE=pcb layout exam job with AI and deterministic checks.

Parallel pcb_status pipeline: parse board, classify domains/groups,
inventory traces, PE-LAY/PLC/SI/DRT checks, per-IC datasheet review.
Findings merge into the report UI. No auto-place or pcbnew write-back.

backend/periscopex/pcb_review.py

@@ -0,0 +1,125 @@
+"""PCB datasheet review prompt and layout neighborhood context (no auto-place)."""
+
+from __future__ import annotations
+
+import math
+
+from backend.periscopex.functional_groups import FunctionalGroupsReport
+from backend.periscopex.models import DesignGraph, LayoutGraph
+from backend.periscopex.pcb_inventory import PcbInventoryReport
+from backend.periscopex.si_check import net_length_mm
+
+PCB_SYSTEM_PROMPT = """\
+You are an electrical engineer reviewing a PCB layout against the IC \
+datasheet. This is an EXAM of the existing board — do not propose a new \
+floorplan, do not invent millimetres, and do not write Gerbers.
+
+### Coverage checklist (layout)
+- Domains (power-rail islands) and functional groups (IC + satellites: \
+decoupling, bulk, filter, crystal, pullup).
+- Placement: decoupling and crystal load caps vs datasheet layout notes \
+and any numeric layout_rules (max_distance_mm, same_layer, thermal vias).
+- Routing: net lengths, differential-pair skew, obvious stubs; length \
+match only when the datasheet gives millimetres.
+- Impedance: comment on Z0 only when stackup + width numbers are in the \
+context. Never assume 50 Ω.
+- Derating: flag a capacitor only when both operating and rated voltages \
+are present and Vop exceeds Vrated.
+- Filters: topology already on the schematic — check whether filter parts \
+sit with the IC they serve when coordinates exist.
+- Datasheet layout pages (typical application / PCB layout) vs this \
+neighborhood.
+
+### Findings
+Every finding MUST have status ERROR, WARNING, or INFO and a non-empty \
+recommendation (what to change on the board). INFO still needs a next \
+step (e.g. "Measure Z0 after stackup is filled in").
+
+Call submit_review. Empty findings with checked_areas is valid when the \
+layout matches the datasheet.
+"""
+
+
+def build_pcb_layout_context(
+    ic_ref: str,
+    graph: DesignGraph,
+    layout: LayoutGraph | None,
+    plan: FunctionalGroupsReport | None,
+    inventory: PcbInventoryReport | None = None,
+) -> str:
+    """Plain-text block appended to the schematic neighborhood for PCB review."""
+    lines = ["### PCB layout context (existing board — do not move parts)"]
+    domain_id = None
+    group = None
+    if plan is not None:
+        for g in plan.groups:
+            if g.ref == ic_ref:
+                group = g
+                break
+        for d in plan.domains:
+            if ic_ref in d.ic_refs:
+                domain_id = d.domain_id
+                lines.append(
+                    f"Domain: {d.domain_id} (power nets: {', '.join(d.power_nets) or '—'})"
+                )
+                break
+        if group is None:
+            lines.append("Functional group: (this IC is not in a classified group)")
+        else:
+            lines.append(
+                f"Functional group: {group.ref} subtype={group.component_subtype or '—'} "
+                f"rank={group.rank}"
+            )
+            sats = group.satellites or []
+            if sats:
+                lines.append("Satellites:")
+                for s in sats:
+                    xy = ""
+                    if layout and s.ref in layout.footprints:
+                        fp = layout.footprints[s.ref]
+                        xy = f" @ ({fp.x:.2f},{fp.y:.2f}) {fp.layer}"
+                    lines.append(
+                        f"  {s.ref} role={s.role_hint} nets={','.join(s.nets or [])}{xy}"
+                    )
+            else:
+                lines.append("Satellites: none")
+    if layout and ic_ref in layout.footprints:
+        fp = layout.footprints[ic_ref]
+        lines.append(
+            f"Footprint {ic_ref}: ({fp.x:.2f},{fp.y:.2f}) layer={fp.layer} "
+            f"pads={len(fp.pads)}"
+        )
+        near: list[str] = []
+        for other, ofp in layout.footprints.items():
+            if other == ic_ref:
+                continue
+            dist = math.hypot(ofp.x - fp.x, ofp.y - fp.y)
+            if dist <= 15.0:
+                near.append(f"{other} {dist:.1f}mm {ofp.layer}")
+        if near:
+            lines.append("Within 15 mm: " + "; ".join(sorted(near)[:24]))
+        pin_nets = graph.components.get(ic_ref)
+        if pin_nets:
+            seen: set[str] = set()
+            lines.append("Connected net lengths (PCB copper):")
+            for net_name in pin_nets.pins.values():
+                if not net_name or net_name in seen:
+                    continue
+                seen.add(net_name)
+                mm = net_length_mm(layout, net_name)
+                lines.append(f"  {net_name}: {mm:.2f} mm")
+    elif layout is None:
+        lines.append("No LayoutGraph — skip millimetre claims.")
+    else:
+        lines.append(f"No footprint for {ic_ref} on the PCB.")
+    if inventory and inventory.nets:
+        sample = inventory.nets[:12]
+        lines.append("Inventory sample (name length_mm pair):")
+        for n in sample:
+            lines.append(
+                f"  {n.name}: {n.length_mm:.2f} mm pair={n.pair or '—'} "
+                f"Z0={n.z0_ohm if n.z0_ohm is not None else '—'}"
+            )
+    if domain_id:
+        lines.append(f"(domain_id={domain_id})")
+    return "\n".join(lines)