Research selective emissive bloom for the HDR client renderer

Investigate a modern pre-tonemap bloom pipeline for PRK.Client that primarily responds to authored emissive content rather than broadly hazing bright scenes. Scope: - Trace the current HDR render target, post-processing, tonemap, transparency, and emissive data paths. - Compare production bloom filtering and selective-emission strategies. - Recommend render-target layout, pass ordering, filters, controls, edge cases, and validation criteria appropriate to the existing Vulkan renderer. Acceptance criteria: - Recommendation is grounded in the current PRK.Client architecture. - Bloom is composed in scene-linear HDR before the SDR display transform. - Design explicitly controls non-emissive scene bloom and avoids haze, halos, flicker, and temporal instability. - Implementation phases and measurable visual/performance tests are identified. Outcome: Recommend a dedicated scene-linear emissive RGB attachment, followed after translucency by a half-resolution Jimenez-style downsample/upsample pyramid. Composite its restrained result before the existing AgX SDR transform and SMAA preparation. Keep sky, ordinary lighting, and UI out by default; preserve the current alpha-masked unlit-material equation when extracting emission; include additive effects explicitly. Use high-quality first downsampling and first-level Karis weighting for stability, clamp/sanitize HDR input, cap broad low-frequency levels, and profile a blendable/filterable B10G11R11 target with RGBA16F fallback. Underwater attenuation must be applied to emission before bloom extraction, not only during final display.