Add a navigable Gradientfall documentation map

[danieltalbert]

What this adds

• a purpose-based documentation map for design, canon, architecture, content authoring, status, and visual evidence
• a compact README navigation row so visitors can reach the map, roadmap, worldbook, architecture, or local run instructions immediately
• current-slice bullets for Bootstrap, the Perceptron Vault, and the Iris classifier compendium
• a corrected current content count (111 approved entries; empty validated inbox)

Why

This applies GitHub's README/navigation guidance without changing the existing art direction or project voice. Gradientfall now has enough design and development material that a dedicated map materially reduces visitor friction.

Validation

• all linked local targets exist
• python tools/validate_content.py --all — PASS (111 entries, 0 errors)
• git diff --check — PASS