[CanReader]

What this changes

CI builds pull requests into staging against SleakEngine staging, and pull requests into main against SleakEngine main. Both branches also require a maintainer's approval.

Why

CI always built against engine main, so a change that needs an engine change could not pass until the engine change had already shipped to every game. SleakEngine now has a staging branch too (CanReader/SleakEngine, ci: route changes through staging).

How it was verified

• Builds on Linux
• Loaded a world and played it
• Placed and broke blocks
• Saved, quit, reloaded, and the world was intact
• Not runtime tested (explain why below)

CI configuration and docs only. The workflow YAML parses; the engine checkouts in all three jobs (build matrix, shaders, smoke test) use the same ENGINE_REF.

Checklist

• Shader changes updated all backend variants and the committed .spv (no shader changes)
• Save format changes bumped WorldMeta::CURRENT_VERSION and handle old worlds (no save changes)
• Collision changes kept the single-pass MTV resolution intact (no collision changes)
• New event handlers are unregistered on scene teardown (no new handlers)