feat: WP-0005 — diagram renderer integration (Mermaid, Graphviz, PlantUML)

Add pluggable DiagramRenderer protocol and RendererResult type to diagrams.py.
Implement MermaidRenderer (mmdc), GraphvizRenderer (dot), PlantUMLRenderer
backends with graceful source-only fallback and WarningRecord on missing tool
(FR-533, FR-534, FR-538). Builder detects renderers at build time and embeds
PNG with alt-text source marker for round-trip. Extend regression corpus with
rendered_diagrams_document.md and skipif-gated integration tests. All 272 tests
pass; ruff and mypy clean.

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

tests/regression/level3/rendered_diagrams_document.md

@@ -0,0 +1,39 @@
+# Document with Rendered Diagrams
+
+This document is used for rendered-diagram round-trip regression (WP-0005 T05).
+Each block exercises a different renderer backend.
+
+## Mermaid State Machine
+
+```mermaid
+stateDiagram-v2
+    [*] --> Idle
+    Idle --> Active: start
+    Active --> Done: complete
+    Done --> [*]
+```
+
+## Graphviz Dependency Graph
+
+```graphviz
+digraph deps {
+    A -> B;
+    A -> C;
+    B -> D;
+    C -> D;
+}
+```
+
+## PlantUML Sequence
+
+```plantuml
+@startuml
+Alice -> Bob: Request
+Bob --> Alice: Response
+@enduml
+```
+
+## Summary
+
+All three diagram types tested in rendered mode when tools are available,
+source-only mode when they are not.