[Feature] Add SkyEnvironment snapshots and fallback lifecycle

[JeanPhilippeKernel]

Type

• Bug
• Feature
• Refactor / cleanup
• Task / chore

Overview

Create the scene sky coordinator and immutable GPU snapshots that guarantee valid environment resources throughout loading, baking, replacement, and failure.

Context

SkySystem coordinates revisions; SkyEnvironment owns persistent resources and publishes fallback, baking, ready, or failed snapshots. Only the render thread may mutate Vulkan state.

What needs to be done

• Implement revision-tagged SkyEnvironment snapshots and a render-thread mailbox for immutable SkyConfig updates.
• Create and publish an always-valid fallback environment before normal sky work.
• Pin selected snapshots for submitted frames and retire them only after timeline completion.
• Coalesce edits and discard stale worker completions.
• Surface actionable bake and fallback diagnostics.

Resources

• ZEngine/docs/future-plan/sky-rendering.md sections 3 and 7

Testing

Test first-frame fallback, rapid edits, stale completion rejection, snapshot replacement, and deferred retirement under in-flight frames.

Acceptance criteria

• No frame binds an invalid environment resource.
• Prior snapshots remain valid until all consuming submissions retire.
• A failed or loading environment preserves a visible fallback.
• Worker threads never write Vulkan descriptors or shared environment state.
• Builds without warnings in Debug and Release.
• All relevant unit tests pass.
• No regression in existing functionality.

Estimated effort

3-5 days.