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_inventory.py

@@ -0,0 +1,97 @@
+"""Trace / bus inventory from a LayoutGraph. Numbers only, no findings."""
+
+from __future__ import annotations
+
+import re
+from collections import defaultdict
+
+from pydantic import BaseModel
+
+from backend.periscopex.models import DesignGraph, LayoutGraph, NetType
+from backend.periscopex.si_check import net_length_mm, partner_net
+
+
+_BUS_RE = re.compile(r"^(.*?)(\d+)$")
+
+
+class PcbNetInventory(BaseModel):
+    name: str
+    length_mm: float = 0.0
+    via_count: int = 0
+    layers: list[str] = []
+    width_min_mm: float | None = None
+    width_max_mm: float | None = None
+    pair: str | None = None
+    bus: str | None = None
+    net_type: str | None = None
+    z0_ohm: float | None = None
+
+
+class PcbInventoryReport(BaseModel):
+    nets: list[PcbNetInventory] = []
+    domains: list[str] = []
+    group_count: int = 0
+
+
+def _bus_name(net: str) -> str | None:
+    m = _BUS_RE.match(net.split("/")[-1].replace(".", "_"))
+    if not m:
+        return None
+    prefix, digits = m.group(1), m.group(2)
+    if not prefix or len(digits) > 3:
+        return None
+    return prefix.rstrip("_") or None
+
+
+def build_pcb_inventory(
+    layout: LayoutGraph,
+    graph: DesignGraph | None = None,
+    *,
+    domain_ids: list[str] | None = None,
+    group_count: int = 0,
+    z0_by_net: dict[str, float] | None = None,
+) -> PcbInventoryReport:
+    names: set[str] = set()
+    layers: dict[str, set[str]] = defaultdict(set)
+    widths: dict[str, list[float]] = defaultdict(list)
+    vias: dict[str, int] = defaultdict(int)
+    for s in layout.segments:
+        if not s.net:
+            continue
+        names.add(s.net)
+        if s.layer:
+            layers[s.net].add(s.layer)
+        if s.width > 0:
+            widths[s.net].append(s.width)
+    for v in layout.vias:
+        if v.net:
+            names.add(v.net)
+            vias[v.net] += 1
+    z0_by_net = z0_by_net or {}
+    rows: list[PcbNetInventory] = []
+    for name in sorted(names):
+        w = widths.get(name) or []
+        sch = graph.nets.get(name) if graph else None
+        nt = sch.net_type.value if sch and isinstance(sch.net_type, NetType) else (
+            str(sch.net_type) if sch and sch.net_type else None
+        )
+        partner = partner_net(name)
+        if partner and partner not in names:
+            partner = None
+        rows.append(PcbNetInventory(
+            name=name,
+            length_mm=round(net_length_mm(layout, name), 3),
+            via_count=vias.get(name, 0),
+            layers=sorted(layers.get(name, ())),
+            width_min_mm=min(w) if w else None,
+            width_max_mm=max(w) if w else None,
+            pair=partner,
+            bus=_bus_name(name),
+            net_type=nt,
+            z0_ohm=z0_by_net.get(name),
+        ))
+    return PcbInventoryReport(
+        nets=rows,
+        domains=list(domain_ids or []),
+        group_count=group_count,
+    )