Add DC-bias C_eff stima to derating and INFO when bulk C has no HF ceramic.

Keep both as labelled estimates: no Murata lot curve and no invented Z(f) target without f_sw.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/pinscopex/hf_coverage_check.py

@@ -0,0 +1,109 @@
+"""HF decoupling coverage — bulk without a small ceramic.
+
+Without a switching frequency this does not invent a Z(f) target.
+INFO only: HF coverage depends on a ~100 nF close to the pin.
+"""
+
+from __future__ import annotations
+
+from backend.pinscopex.models import ComponentType, DesignGraph, Finding, NetType
+from backend.pinscopex.passive_rail_check import (
+    _cap_farads,
+    _is_ground_net,
+    _is_ic_supply_pin,
+    _is_nc_net,
+    _is_regulator_output_pin,
+    _pin_label,
+)
+from backend.pinscopex.validate import _match_constraints
+
+_BULK_MIN_F = 1e-6
+_HF_MAX_F = 1e-6
+_HF_MIN_F = 1e-9
+
+
+def _esl_hint(footprint: str) -> str:
+    fp = (footprint or "").upper()
+    if "0402" in fp:
+        return "typical ESL ~0.4 nH (0402 stima)"
+    if "0603" in fp:
+        return "typical ESL ~0.6 nH (0603 stima)"
+    if "0805" in fp:
+        return "typical ESL ~0.8 nH (0805 stima)"
+    return "ESL depends on package (stima)"
+
+
+def _valued_gnd_caps(graph: DesignGraph, net_name: str) -> list[tuple[str, float]]:
+    out: list[tuple[str, float]] = []
+    unknown = False
+    for ref in graph.capacitors_on_net(net_name):
+        cap = graph.components[ref]
+        others = {n for n in cap.pins.values() if n != net_name}
+        if not any(_is_ground_net(graph, n) for n in others):
+            continue
+        farads = _cap_farads(cap)
+        if farads is None:
+            unknown = True
+            continue
+        out.append((ref, farads))
+    if unknown:
+        return []
+    return out
+
+
+def check_hf_decoupling_coverage(
+    graph: DesignGraph,
+    constraints_map: dict,
+) -> list[Finding]:
+    findings: list[Finding] = []
+    seen: set[str] = set()
+    for ref, comp in sorted(graph.components.items()):
+        if comp.component_type != ComponentType.IC:
+            continue
+        cons = _match_constraints(comp.mpn or comp.value, constraints_map)
+        for pin_num, net_name in sorted(comp.pins.items(), key=lambda x: str(x[0])):
+            if net_name in seen or _is_nc_net(net_name):
+                continue
+            is_rail = _is_ic_supply_pin(graph, cons, pin_num, net_name) or (
+                _is_regulator_output_pin(cons, pin_num)
+            )
+            if not is_rail:
+                continue
+            net = graph.nets.get(net_name)
+            if net and net.net_type == NetType.GROUND:
+                continue
+            seen.add(net_name)
+            caps = _valued_gnd_caps(graph, net_name)
+            if not caps:
+                continue
+            has_bulk = any(c >= _BULK_MIN_F for _, c in caps)
+            has_hf = any(_HF_MIN_F <= c < _HF_MAX_F for _, c in caps)
+            if not (has_bulk and not has_hf):
+                continue
+            bulk_ref = next(r for r, c in caps if c >= _BULK_MIN_F)
+            fp = graph.components[bulk_ref].footprint
+            pin_label = _pin_label(cons, pin_num, net_name)
+            findings.append(Finding(
+                designator=ref,
+                mpn=comp.mpn or "",
+                aspect="decoupling",
+                source="hf_coverage_check",
+                status="INFO",
+                finding=(
+                    f"{ref} net '{net_name}' ({pin_label}) has bulk capacitance "
+                    f"but no ~100 nF ceramic for HF."
+                ),
+                why=(
+                    f"Parallel Z(f) of large C is inductive above a few hundred "
+                    f"kHz ({_esl_hint(fp)}). Without f_sw this is not an Ω target."
+                ),
+                recommendation=(
+                    f"Add a 10–100 nF ceramic from '{net_name}' to ground near "
+                    f"{ref}, in parallel with the bulk cap."
+                ),
+                reference="netlist topology (stima)",
+                net=net_name,
+                pins=[f"{ref}.{pin_num}"],
+                rule_id="PS-ESR-001",
+            ))
+    return findings