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>

tests/regression/level3/figures_document.md

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+# Technical Report with Figures
+
+## Overview
+
+This document contains multiple numbered figures for LEVEL3 round-trip testing.
+
+## System Architecture
+
+The overall architecture is illustrated below.
+
+![System Architecture Overview](figures/architecture.png){#fig:arch}
+
+The architecture shows the main components and their interactions.
+
+## Data Flow
+
+The data flow is shown in the following figure.
+
+![Data Flow Diagram](figures/dataflow.png){#fig:dataflow}
+
+Compare the architecture in [fig:arch] with the data flow above.
+
+## Results
+
+Final results are captured in this chart.
+
+![Results Summary Chart](figures/results.png){#fig:results}
+
+The chart confirms the findings from the data flow in Figure 2.