docs: migrate all wiki pages to Mermaid diagrams + add Asset Manager page
- Engine-Architecture: Big Picture, cross-thread comms (mailbox/MTS/RRM),
WorldTick wave execution, WorldCommands merge, asset pipeline, VFS hot-reload,
init sequence, and shutdown order all converted to Mermaid flowchart/sequence
- Memory-Management: memory budget hierarchy graph, scratch arena sequence,
deferred Vulkan destroy flow, GPU domain graph — all Mermaid; budget table
updated to reflect new 8 GB root and open-world sizes (#635)
- Rendering-Domain: architecture overview, per-frame sequence, scene mesh
pipeline, material pipeline, RGS pass DAG, compaction sequence, shutdown
order — all converted to Mermaid; recently-fixed table updated with #632-634
- Asset-Manager: new page covering pipeline position, memory layout, all four
ingest flows, GPU material binding end-to-end, lookup API, thread safety,
initialization contract, and known gaps
- Home: added Asset Manager entry to the page index