Harden renderer lifecycle, frame flow, resize, backend selection and recovery (Statement 9)

[ercmine]

Motivation

• Make the renderer runtime robust and production-minded by owning startup/shutdown, backend selection, swapchain/frame lifecycle, resizing, pacing and a pragmatic recovery path.
• Replace scattered booleans with an explicit engine-owned lifecycle model so future systems can query renderability and not fight init/resize ordering.
• Ensure the runtime shell uses a disciplined begin/end frame contract and that resize and device-loss paths are deterministic and well-logged.

Description

• Added explicit lifecycle types and status snapshots via RendererLifecycleState, RendererFrameStats, RendererStatus, and RendererConfigValidation, and utility helpers like is_explicit_backend_request and string converters; exposed new query APIs state(), status(), is_ready(), and can_render().
• Hardened startup with validate_renderer_config, deterministic engine-owned backend mapping (RendererBackend) with requested-vs-actual logging, capability logging, and guarded re-init/shutdown transitions.
• Reworked frame contract to guarded begin_frame() / end_frame() that detect misuse (double-begin, end-without-begin), apply deferred/deduplicated request_resize() at frame boundary, and record lightweight per-frame timing stats.
• Implemented safe resize handling that defers zero/minimized dimensions, deduplicates apply requests, and updates backbuffer via bgfx::reset without requiring an app restart.
• Introduced a lightweight frame pacing floor via RendererConfig::min_frame_time_ms applied in end_frame() (no simulation scheduler added).
• Added a pragmatic recovery path with try_recover() and Recovering/Failed states that attempts in-place reset and logs outcomes, preparing the architecture for future full resource re-creation.
• Integrated runtime shell changes to forward platform resize events with request_resize() and to gate rendering on begin_frame() success, and updated documentation and engine README to describe the lifecycle contract.
• Added a small unit test exercising config validation and lifecycle pre-init behavior and wired it into CTest (render_renderer_lifecycle_tests) so CI can cover lifecycle regressions once dependencies are available.

Testing

• Added unit test tests/render/renderer_lifecycle_tests.cpp and registered the unit.renderer.lifecycle CTest target so lifecycle checks are included in render_test_unit.
• Attempted configure with cmake --preset linux-debug in this environment but configure failed due to unresolved third-party dependency roots for SDL3/bgfx, so the project could not be built or tests executed here.
• The new unit test contains only header-level/runtime-safety assertions and will run in CI once dependencies are provided; configuration failures are environmental and not caused by the lifecycle changes.

---

Codex Task