Port the first retail precipice-slide slice from named retail/ACE: terrain and BSP walkable hits now preserve polygon vertices, failed step-down edges back-probe to rediscover the walkable polygon, and edge-slide can run precipice/cliff slide instead of only hard-stopping.
Adds pseudocode anchors plus regression coverage for terrain polygon context and loaded-terrain boundary edge-slide.
Co-authored-by: Codex <codex@openai.com>
