Add a deterministic SVG KV-retention visualization

[DaBestCode]

Create a small script that renders one eviction decision as a standalone SVG for documentation and launch materials.

Acceptance criteria:

• Accept budget, buffer_size, prompt_length, generated-token count, and seed.
• Visually distinguish protected prompt tokens, randomly retained generated tokens, recent-buffer tokens, and evicted tokens.
• Produce byte-identical output for identical inputs.
• Use the existing RandomEvictionPolicy; do not reimplement selection logic.
• Add a focused test and a short usage example.
• Keep the core package dependency-free.