Feat: Add optional three-channel sensor bloom model

Add an opt-in, post-processing limited-response model applied to the
finished linear HDR before tone mapping. Each RGB channel is processed
independently and isotropically: overflow above E spreads to the eight
neighbours with a fixed 9-point stencil, while the rest is absorbed or
lost at the image boundary. The synchronous ping-pong update uses a
monotonic bounding box and a row-parallel, deterministic reduction; the
conservative round bound reserves fp guard rounds inside a 4096 hard
limit and fails before touching HDR when exceeded.

Expose --sensor-bloom-limit E and --sensor-bloom-transfer e (both
required together, default disabled), validate them before expensive
initialization, and route every output path through the same hook in
write_frame_outputs: raw FITS first, bloom, tone-mapped PNG/PPM, then the
mesh overlay. The raw --hdr-output FITS therefore stays pre-bloom.

Add a standalone unit test (stencil, boundary loss, cascade reference,
symmetry, thread determinism, convergence limits, validation, allocation
failure), CLI integration and regression coverage, an isolated
sensor-bloom-bench target, and document the model in the design, usage,
README, and build docs.

README.zh-CN.md

@@ -86,7 +86,7 @@ mkdir -p output/imgs
 
 这里使用更新后的参考图像的渲染设置。`--max-cache-psf-flux 1e8` 允许将明亮 PSF 的翼部截断在缓存半径内，但本次运行没有发生翼部裁剪或 direct fallback。[CPU/HIP benchmark 记录](benchmarks/2mass_galactic_center_cpu_hip_2026-09-07.md)保留了命令、终端输出，以及 CPU/HIP PNG 比特级一致的结果；上述命令省略了可选的 HDR 和 lens-map 导出。
 
-上述 Schwarzschild 示例省略位置，因此按指向与半径推导出朝向黑洞的相机。自适应细分默认关闭，应根据所需图像精度配置。tone-mapped PNG/PPM 默认使用逐通道 soft-clip 显示变换（`--tone-map softclip --tone-map-p 2`）；`--tone-map reinhard` 恢复上方参考图像使用的历史曲线，线性 HDR FITS 输出不受其影响。相机控制、图像序列、透镜映射复用、PSF 设置及 HDR 输出参见 [usage.md](usage.md)（英文）。两个可执行文件都可通过 `--help` 查看完整选项。
+上述 Schwarzschild 示例省略位置，因此按指向与半径推导出朝向黑洞的相机。自适应细分默认关闭，应根据所需图像精度配置。tone-mapped PNG/PPM 默认使用逐通道 soft-clip 显示变换（`--tone-map softclip --tone-map-p 2`）；`--tone-map reinhard` 恢复上方参考图像使用的历史曲线，线性 HDR FITS 输出不受其影响。可选的有限响应 sensor bloom（`--sensor-bloom-limit E --sensor-bloom-transfer e`，默认关闭）在显示前作用于线性 HDR，且不会改变 FITS 输出。相机控制、图像序列、透镜映射复用、PSF 设置及 HDR 输出参见 [usage.md](usage.md)（英文）。两个可执行文件都可通过 `--help` 查看完整选项。
 
 ### 示例：紧贴史瓦西视界向外看