Add event-taxonomy package with canonical schema, adapters, and CLI

Canonical NormalizedFinding schema with Severity enum (CRITICAL/HIGH/MEDIUM/LOW/INFO).
Per-project adapters for 9 tools with severity mapping for string labels, int 1-10,
float 0-1, Python Enum, and computed properties. CLI pipe interface and registry.

Nightshift-Task: event-taxonomy
Nightshift-Ref: https://github.com/marcus/nightshift

tests/test_schema.py

@@ -0,0 +1,52 @@
+import json
+
+from event_taxonomy.schema import NormalizedFinding, Severity, ToolEvent
+
+
+def test_severity_ordering():
+    assert Severity.CRITICAL > Severity.HIGH > Severity.MEDIUM > Severity.LOW > Severity.INFO
+
+
+def test_severity_str():
+    assert str(Severity.CRITICAL) == "CRITICAL"
+    assert str(Severity.INFO) == "INFO"
+
+
+def test_finding_construction():
+    f = NormalizedFinding(
+        tool="test-tool",
+        category="test",
+        severity=Severity.HIGH,
+        message="something broke",
+        file="foo.py",
+        line=42,
+    )
+    assert f.tool == "test-tool"
+    assert f.severity == Severity.HIGH
+    assert f.file == "foo.py"
+    assert f.line == 42
+    assert f.metadata == {}
+    assert f.recommendation is None
+
+
+def test_finding_to_dict():
+    f = NormalizedFinding(tool="t", category="c", severity=Severity.LOW, message="m")
+    d = f.to_dict()
+    assert d["severity"] == "LOW"
+    assert d["tool"] == "t"
+
+
+def test_tool_event_serialization():
+    findings = [
+        NormalizedFinding(tool="t", category="c", severity=Severity.HIGH, message="a"),
+        NormalizedFinding(tool="t", category="c", severity=Severity.LOW, message="b"),
+    ]
+    event = ToolEvent(tool_name="t", tool_version="1.0", findings=findings)
+    d = event.to_dict()
+    assert d["summary"]["total"] == 2
+    assert d["summary"]["HIGH"] == 1
+    assert d["summary"]["LOW"] == 1
+    j = event.to_json()
+    parsed = json.loads(j)
+    assert parsed["tool_name"] == "t"
+    assert len(parsed["findings"]) == 2