feat: complete compact far-terrain rendering

[MichaelFisher1997]

Summary

• complete the compact LOD3/4 tile, pooled upload, reusable-grid rendering, GPU-culling, persistence, telemetry, and benchmark work for the Phase 4/5 far-terrain roadmap
• add compact terrain/water shaders, ABI and bounds checks, Phase 5 policy/visual gates, SPIR-V baselines, and focused repository guidance
• fail ZIGCRAFT_LOD_COMPACT=auto closed to expanded GPU meshes after reproducing RADV VK_ERROR_DEVICE_LOST; retain force only for bounded diagnostics and keep exclusive-buffer uploads on the graphics queue until ownership transfers exist

Validation

• nix develop --command zig fmt src/ modules/
• nix develop --command zig build test (438/438)
• nix develop --command zig build -Doptimize=ReleaseFast
• nix develop --command zig build test-integration -Dskip-present
• nix develop --command zig build phase5-gate
• nix develop --command zig build phase5-visual-gate
• bounded visible saved-world RADV verification: default fail-closed path stable

Known limitation

Compact/mixed rendering remains unsafe on the tested RADV saved-world workload despite clean Vulkan validation. The production default therefore uses expanded GPU LOD meshes; ZIGCRAFT_LOD_COMPACT=force is diagnostic only.

Related to #917.

[MichaelFisher1997]

Closing because this incremental Phase 5 work belongs on the stage integration branch. A replacement PR targeting stage will follow.

[github-actions]

APIError: You've reached your usage limit for this billing cycle. Your quota will be refreshed in the next cycle. To continue now, purchase extra usage or upgrade your plan: https://www.kimi.com/membership/subscription?tab=quota

opencode session  |  github run