Fix two regressions from PR #88 (rotate/mirror)

1. add_schematic_net_label failed on schematics with no existing labels.
   The clone-based path required a pre-existing label to copy from;
   the documented "fallback to sexpdata" was a misleading log line —
   the RuntimeError was caught and the call silently returned False.
   Restore hand-built sexpdata construction (without the buggy
   fields_autoplaced token, with orientation-aware justify).

2. get_pin_angle returned the wrong angle for every mirrored symbol
   (off by exactly 180°, all rotations, both mirror axes). The
   mirror_x and mirror_y formulas were swapped relative to the
   pin_world_xy convention — pin_world_xy mirrors a position by
   flipping its local axis component, so the matching angle
   transform is (180 - θ) for mirror_x and -θ for mirror_y.

Add regression tests:
- test_add_label_empty_schematic.py — first label on empty schematic,
  orientation-aware justify.
- test_get_pin_angle.py — full 24-case matrix
  (4 rotations × 3 mirror states × 2 pins).

tests/test_add_label_empty_schematic.py

@@ -0,0 +1,105 @@
+"""
+Regression tests for WireManager.add_label on schematics with zero existing labels.
+
+Bug: PR #88 rewrote add_label to clone an existing label via kicad-skip; on a
+fresh schematic with no labels this returns False. The fix restores a hand-built
+sexpdata path (without the spurious fields_autoplaced token) as the primary path.
+"""
+
+import importlib.util
+import os
+import sys
+from unittest.mock import MagicMock
+
+import sexpdata
+from sexpdata import Symbol
+
+# Stub heavy / optional deps before loading wire_manager
+for modname in ("pcbnew", "skip"):
+    sys.modules.setdefault(modname, MagicMock())
+
+_wm_spec = importlib.util.spec_from_file_location(
+    "wire_manager",
+    os.path.join(os.path.dirname(__file__), "..", "python", "commands", "wire_manager.py"),
+)
+_wm_mod = importlib.util.module_from_spec(_wm_spec)
+_wm_spec.loader.exec_module(_wm_mod)
+WireManager = _wm_mod.WireManager
+
+
+_EMPTY_SCH = """\
+(kicad_sch (version 20250114) (generator "test")
+  (lib_symbols)
+  (sheet_instances
+    (path "/" (page "1"))
+  )
+)
+"""
+
+
+def _find_label(sch_data, text):
+    for item in sch_data:
+        if (
+            isinstance(item, list)
+            and len(item) >= 2
+            and isinstance(item[0], Symbol)
+            and str(item[0]) == "label"
+            and item[1] == text
+        ):
+            return item
+    return None
+
+
+def _has_token(label_sexp, name):
+    for part in label_sexp[2:]:
+        if isinstance(part, list) and part and isinstance(part[0], Symbol) and str(part[0]) == name:
+            return True
+    return False
+
+
+def _find_subtoken(label_sexp, name):
+    for part in label_sexp[2:]:
+        if isinstance(part, list) and part and isinstance(part[0], Symbol) and str(part[0]) == name:
+            return part
+    return None
+
+
+def test_add_label_to_empty_schematic_succeeds(tmp_path):
+    sch_path = tmp_path / "empty.kicad_sch"
+    sch_path.write_text(_EMPTY_SCH)
+
+    ok = WireManager.add_label(sch_path, "TEST", [50.0, 50.0], "label", 0)
+    assert ok is True
+
+    sch_data = sexpdata.loads(sch_path.read_text())
+    label = _find_label(sch_data, "TEST")
+    assert label is not None, 'Expected a (label "TEST" ...) token in the file'
+
+    # The fields_autoplaced token must NOT be present (regression guard for PR #88's bug).
+    assert not _has_token(
+        label, "fields_autoplaced"
+    ), "fields_autoplaced should not be emitted by add_label"
+
+
+def test_add_label_orientation_180_uses_right_bottom_justify(tmp_path):
+    sch_path = tmp_path / "empty180.kicad_sch"
+    sch_path.write_text(_EMPTY_SCH)
+
+    ok = WireManager.add_label(sch_path, "NETR", [60.0, 60.0], "label", 180)
+    assert ok is True
+
+    sch_data = sexpdata.loads(sch_path.read_text())
+    label = _find_label(sch_data, "NETR")
+    assert label is not None
+
+    effects = _find_subtoken(label, "effects")
+    assert effects is not None, "label must carry an (effects ...) token"
+
+    justify = _find_subtoken(effects, "justify")
+    assert justify is not None, "effects must carry a (justify ...) token"
+
+    justify_vals = [str(t) for t in justify[1:] if isinstance(t, Symbol)]
+    assert justify_vals == [
+        "right",
+        "bottom",
+    ], f"orientation=180 should produce 'right bottom' justify, got {justify_vals}"