feat: WP-0003 complete — LEVEL3 advanced features + error framework

Implements full LEVEL3 feature set: cross-references (xref.py), numbered
figures (figures.py), auto-diagrams (diagrams.py), bibliography/citations
(bibliography.py), LEVEL3 capability detection (level3.py), and structured
error/warning records (errors.py). Builder, importer, and differ updated for
LEVEL3 round-trip support. REST and MCP interfaces updated with structured
warning records. 259 tests passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/markidocx/errors.py

@@ -0,0 +1,80 @@
+"""Structured error and warning types for markidocx (FR-1201–1210)."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import StrEnum
+from typing import Any
+
+
+class Severity(StrEnum):
+    INFO = "info"
+    WARNING = "warning"
+    ERROR = "error"
+
+
+class OutputState(StrEnum):
+    """Lifecycle state of a build/import/workflow result (FR-1210)."""
+
+    FINAL = "final"
+    PARTIAL = "partial"
+    FALLBACK = "fallback"
+    DEGRADED = "degraded"
+    UNRESOLVED = "unresolved"
+
+
+@dataclass
+class WarningRecord:
+    """Structured warning record (FR-1208).
+
+    severity: info | warning | error
+    reason:   FR-code-aligned description
+    construct: the token/element that triggered the warning
+    """
+
+    severity: str
+    reason: str
+    construct: str = ""
+
+    def to_dict(self) -> dict[str, Any]:
+        return {
+            "severity": self.severity,
+            "reason": self.reason,
+            "construct": self.construct,
+        }
+
+    def __str__(self) -> str:
+        if self.construct:
+            return f"[{self.severity}] {self.reason}: {self.construct}"
+        return f"[{self.severity}] {self.reason}"
+
+
+@dataclass
+class FailureRecord:
+    """Structured failure record (FR-1209).
+
+    severity: info | warning | error
+    reason:   FR-code-aligned description
+    construct: the element that caused the failure
+    """
+
+    severity: str
+    reason: str
+    construct: str = ""
+
+    def to_dict(self) -> dict[str, Any]:
+        return {
+            "severity": self.severity,
+            "reason": self.reason,
+            "construct": self.construct,
+        }
+
+    def __str__(self) -> str:
+        if self.construct:
+            return f"[{self.severity}] {self.reason}: {self.construct}"
+        return f"[{self.severity}] {self.reason}"
+
+
+def warning_records_to_strings(records: list[WarningRecord]) -> list[str]:
+    """Convert a list of WarningRecords to plain strings (backward compat helper)."""
+    return [str(r) for r in records]