rendering: add retained render layers and replay lifecycle

[tritao]

Builds on #54 and completes the retained renderer architecture before the performance follow-up stack.

Scope

• stage-aware render layers and scoped depth clears
• delayed traversal with captured replay context
• backend shutdown versus context-loss discard
• authoritative render-plan ordering for drawing, selection, and picking
• frame-local selection ownership and visual-depth-segment picking
• public renderer capability and matrix-policy contracts

This PR owns renderer semantics and lifecycle, not benchmark infrastructure or performance optimizations. Those begin in #67.

Validation

The complete descendant stack is exercised in core-only and compatibility configurations. This layer also keeps implementation-only GL fixtures out of Windows shared-library builds, where private retained symbols are intentionally not exported.