Add deterministic workflow visualization export

[UntitledPhases]

Summary

• adds deterministic JSON, Mermaid, and SVG visualization exports for workflow evidence
• wires visualization into the CLI with validation-aware exit behavior
• documents the graph model and output contract
• adds a complete readiness-workflow evidence fixture and generated SVG
• adds visualization-focused tests

Why

This feature was complete locally but unpublished. The draft PR preserves the implementation and its evidence bundle without changing the default branch.

Impact

Adds a reviewable visualization layer while preserving the validator as the source of truth.

Checks

• py -3.13 -m unittest discover -s tests -v — 17 passed
• git diff --cached --check — passed
• high-confidence secret and personal-context scan — clean