fix(add_schematic_wire): support hierarchical sub-sheets (#170)

WireManager.add_wire and add_polyline_wire bailed out with
"No sheet_instances section found in schematic" when called on a
hierarchical sub-sheet — that block only exists in the root .kicad_sch.
The handler in kicad_interface.py converted the False return into a
flat "Failed to add wire" message, leaving callers with no diagnostic.

Apply the same fallback that WireManager.add_label has used since the
hierarchical-design support: when (sheet_instances ...) is absent,
append the new (wire ...) item at the end of the outer (kicad_sch ...)
form.

Adds tests/test_add_wire_sub_sheet.py with 6 regression tests covering
both add_wire and add_polyline_wire on a sub-sheet, including paren
balance, sexpdata round-trip, and segment count for an N-point polyline.

python/commands/wire_manager.py

@@ -174,7 +174,9 @@ class WireManager:
                 start_point, end_point, stroke_width, stroke_type
             )
 
-            # Find insertion point (before sheet_instances)
+            # Find insertion point (before sheet_instances on the root sheet,
+            # or appended to the end on a hierarchical sub-sheet which has no
+            # sheet_instances block).
             sheet_instances_index = None
             for i, item in enumerate(sch_data):
                 if isinstance(item, list) and len(item) > 0 and item[0] == _SYM_SHEET_INSTANCES:
@@ -182,10 +184,10 @@ class WireManager:
                     break
 
             if sheet_instances_index is None:
-                logger.error("No sheet_instances section found in schematic")
-                return False
+                # Sub-sheets in hierarchical designs don't have (sheet_instances).
+                sheet_instances_index = len(sch_data)
 
-            # Insert wire before sheet_instances
+            # Insert wire before sheet_instances (or at end for sub-sheets)
             sch_data.insert(sheet_instances_index, wire_sexp)
             logger.info(f"Injected wire from {start_point} to {end_point}")
 
@@ -249,7 +251,9 @@ class WireManager:
                 for i in range(len(points) - 1)
             ]
 
-            # Find insertion point
+            # Find insertion point (before sheet_instances on the root sheet,
+            # or appended to the end on a hierarchical sub-sheet which has no
+            # sheet_instances block).
             sheet_instances_index = None
             for i, item in enumerate(sch_data):
                 if isinstance(item, list) and len(item) > 0 and item[0] == _SYM_SHEET_INSTANCES:
@@ -257,8 +261,8 @@ class WireManager:
                     break
 
             if sheet_instances_index is None:
-                logger.error("No sheet_instances section found in schematic")
-                return False
+                # Sub-sheets in hierarchical designs don't have (sheet_instances).
+                sheet_instances_index = len(sch_data)
 
             # Insert all segments (in reverse so order is preserved after inserts)
             for wire_sexp in reversed(wire_sexps):

tests/test_add_wire_sub_sheet.py

@@ -0,0 +1,135 @@
+"""
+Regression tests for WireManager.add_wire / add_polyline_wire on hierarchical
+sub-sheets.
+
+Sub-sheets do not carry a (sheet_instances ...) block — that block is only
+emitted on the root .kicad_sch in a hierarchical design. Before the fix, both
+methods bailed out with "No sheet_instances section found in schematic" and
+returned False, so users saw "Failed to add wire" on every call into a child
+sheet.
+"""
+
+import sys
+from pathlib import Path
+from typing import Any
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "python"))
+
+
+# Minimal sub-sheet content: same outer (kicad_sch ...) form as a root sheet
+# but WITHOUT (sheet_instances ...).
+SUB_SHEET_NO_SHEET_INSTANCES = """(kicad_sch
+\t(version 20260306)
+\t(generator "eeschema")
+\t(generator_version "10.0")
+\t(uuid "bbbb2222-2222-2222-2222-bbbbbbbbbbbb")
+\t(paper "A4")
+\t(lib_symbols)
+)
+"""
+
+
+def _write_sub_sheet(tmp_path: Path) -> Path:
+    sch = tmp_path / "child.kicad_sch"
+    sch.write_text(SUB_SHEET_NO_SHEET_INSTANCES, encoding="utf-8")
+    return sch
+
+
+@pytest.mark.unit
+class TestAddWireSubSheet:
+    """WireManager.add_wire must succeed on hierarchical sub-sheets."""
+
+    def setup_method(self) -> None:
+        from commands.wire_manager import WireManager
+
+        self.WireManager = WireManager
+
+    def test_add_wire_succeeds_on_sub_sheet(self, tmp_path: Any) -> None:
+        sch = _write_sub_sheet(tmp_path)
+
+        ok = self.WireManager.add_wire(sch, [40.0, 40.0], [40.0, 50.0])
+
+        assert ok is True
+        content = sch.read_text(encoding="utf-8")
+        # The sexpdata writer emits the wire form somewhere in the file.
+        assert "wire" in content
+        assert "40.0" in content or "40 " in content
+
+    def test_add_wire_keeps_outer_form_balanced_on_sub_sheet(self, tmp_path: Any) -> None:
+        sch = _write_sub_sheet(tmp_path)
+
+        self.WireManager.add_wire(sch, [10.0, 10.0], [10.0, 20.0])
+
+        content = sch.read_text(encoding="utf-8")
+        assert content.count("(") == content.count(
+            ")"
+        ), "Inserting into a sub-sheet must keep parens balanced"
+
+        import sexpdata
+
+        parsed = sexpdata.loads(content)
+        assert parsed[0] == sexpdata.Symbol("kicad_sch")
+
+    def test_add_wire_writes_endpoints_on_sub_sheet(self, tmp_path: Any) -> None:
+        """The new wire must round-trip through sexpdata with the requested endpoints."""
+        import sexpdata
+
+        sch = _write_sub_sheet(tmp_path)
+
+        self.WireManager.add_wire(sch, [12.5, 22.5], [42.5, 22.5])
+
+        parsed = sexpdata.loads(sch.read_text(encoding="utf-8"))
+        wires = [
+            item
+            for item in parsed[1:]
+            if isinstance(item, list) and len(item) > 0 and item[0] == sexpdata.Symbol("wire")
+        ]
+        assert wires, "expected at least one (wire ...) item at top level after insert"
+        # Find xy points among the wire's children
+        wire_text = sexpdata.dumps(wires[0])
+        assert "12.5" in wire_text and "22.5" in wire_text and "42.5" in wire_text
+
+
+@pytest.mark.unit
+class TestAddPolylineWireSubSheet:
+    """WireManager.add_polyline_wire must succeed on hierarchical sub-sheets."""
+
+    def setup_method(self) -> None:
+        from commands.wire_manager import WireManager
+
+        self.WireManager = WireManager
+
+    def test_add_polyline_wire_succeeds_on_sub_sheet(self, tmp_path: Any) -> None:
+        sch = _write_sub_sheet(tmp_path)
+
+        ok = self.WireManager.add_polyline_wire(sch, [[10.0, 10.0], [20.0, 10.0], [20.0, 30.0]])
+
+        assert ok is True
+
+    def test_add_polyline_wire_writes_n_minus_1_segments(self, tmp_path: Any) -> None:
+        """A 4-point polyline must insert 3 (wire ...) segments into the file."""
+        import sexpdata
+
+        sch = _write_sub_sheet(tmp_path)
+
+        self.WireManager.add_polyline_wire(
+            sch, [[0.0, 0.0], [10.0, 0.0], [10.0, 10.0], [20.0, 10.0]]
+        )
+
+        parsed = sexpdata.loads(sch.read_text(encoding="utf-8"))
+        wires = [
+            item
+            for item in parsed[1:]
+            if isinstance(item, list) and len(item) > 0 and item[0] == sexpdata.Symbol("wire")
+        ]
+        assert len(wires) == 3, f"expected 3 wire segments, got {len(wires)}"
+
+    def test_add_polyline_wire_keeps_outer_form_balanced_on_sub_sheet(self, tmp_path: Any) -> None:
+        sch = _write_sub_sheet(tmp_path)
+
+        self.WireManager.add_polyline_wire(sch, [[0.0, 0.0], [10.0, 0.0], [10.0, 10.0]])
+
+        content = sch.read_text(encoding="utf-8")
+        assert content.count("(") == content.count(")")