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.

Makefile

@@ -36,7 +36,7 @@ TARGET_BASENAME := $(SPACETIME)_sky
 OBJECT_DIR := $(BUILD_DIR)/obj/$(SPACETIME)
 CORE_MINKOWSKI_SOURCES := $(COMMON_SOURCES) src/spacetime_minkowski.c
 
-.PHONY: all backend clean run test tone-map-test hip-psf-test hip-psf-bench fast-psf-fftw-bench minkowski schwarzschild FORCE
+.PHONY: all backend clean run test tone-map-test sensor-bloom-test sensor-bloom-bench hip-psf-test hip-psf-bench fast-psf-fftw-bench minkowski schwarzschild FORCE
 
 ifneq ($(filter 0 1,$(PSF_EVENT_SINK)),$(PSF_EVENT_SINK))
 $(error Unknown PSF_EVENT_SINK '$(PSF_EVENT_SINK)'; choose 0 or 1)
@@ -114,6 +114,8 @@ CAMERA_TEST_TARGETS := $(TEST_OUT_DIR)/test_observer_minkowski $(TEST_OUT_DIR)/t
 FAST_PSF_FFTW_TEST_TARGET := $(TEST_OUT_DIR)/test_fast_psf_fftw
 FAST_PSF_FFTW_BENCH_TARGET := $(TEST_OUT_DIR)/benchmark_fast_psf_fftw
 TONE_MAP_TEST_TARGET := $(TEST_OUT_DIR)/test_tone_map
+SENSOR_BLOOM_TEST_TARGET := $(TEST_OUT_DIR)/test_sensor_bloom
+SENSOR_BLOOM_BENCH_TARGET := $(TEST_OUT_DIR)/benchmark_sensor_bloom
 
 ifeq ($(PSF_BACKEND),hip)
 TARGET := $(BUILD_DIR)/$(TARGET_BASENAME)_hip
@@ -224,6 +226,15 @@ $(TONE_MAP_TEST_TARGET): tests/test_tone_map.c src/optics.c src/optics.h $(CPU_F
 $(FAST_PSF_FFTW_BENCH_TARGET): tests/benchmark_fast_psf_fftw.c $(CORE_MINKOWSKI_SOURCES) $(CPU_FFTW_SOURCES) | $(TEST_OUT_DIR)
 	$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc $^ $(LDLIBS) -o $@
 
+# The sensor-bloom checks link only the standalone model, so they need neither
+# a catalog nor ray tracing, and build in every ENABLE_HDR/PSF_BACKEND
+# configuration.
+$(SENSOR_BLOOM_TEST_TARGET): tests/test_sensor_bloom.c src/sensor_bloom.c src/sensor_bloom.h | $(TEST_OUT_DIR)
+	$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc tests/test_sensor_bloom.c src/sensor_bloom.c $(LDLIBS) -o $@
+
+$(SENSOR_BLOOM_BENCH_TARGET): tests/benchmark_sensor_bloom.c src/sensor_bloom.c src/sensor_bloom.h | $(TEST_OUT_DIR)
+	$(CC) $(CPPFLAGS) $(BUILD_CPPFLAGS) $(CFLAGS) $(BUILD_CFLAGS) $(OPENMP_FLAGS) -Isrc tests/benchmark_sensor_bloom.c src/sensor_bloom.c $(LDLIBS) -o $@
+
 # The FFTW-vs-spatial test is meaningful only in the CPU PSF build.
 ifneq ($(CPU_FFTW_SOURCES),)
 FAST_PSF_FFTW_TEST_DEP := $(FAST_PSF_FFTW_TEST_TARGET)
@@ -233,7 +244,7 @@ FAST_PSF_FFTW_TEST_DEP :=
 FAST_PSF_FFTW_TEST_RUN :=
 endif
 
-test: $(CAMERA_TEST_TARGETS) $(TEST_TARGET) $(FRAME_TEST_TARGET) $(SCHWARZSCHILD_TEST_TARGET) $(OBSERVER_TRACK_TEST_TARGET) $(CATALOG_PREFETCH_TEST_TARGET) $(FAST_PSF_FFTW_TEST_DEP) $(TONE_MAP_TEST_TARGET)
+test: $(CAMERA_TEST_TARGETS) $(TEST_TARGET) $(FRAME_TEST_TARGET) $(SCHWARZSCHILD_TEST_TARGET) $(OBSERVER_TRACK_TEST_TARGET) $(CATALOG_PREFETCH_TEST_TARGET) $(FAST_PSF_FFTW_TEST_DEP) $(TONE_MAP_TEST_TARGET) $(SENSOR_BLOOM_TEST_TARGET)
 	$(TEST_OUT_DIR)/test_observer_minkowski
 	$(TEST_OUT_DIR)/test_observer_schwarzschild
 	$(TEST_TARGET)
@@ -243,11 +254,17 @@ test: $(CAMERA_TEST_TARGETS) $(TEST_TARGET) $(FRAME_TEST_TARGET) $(SCHWARZSCHILD
 	$(CATALOG_PREFETCH_TEST_TARGET)
 	$(FAST_PSF_FFTW_TEST_RUN)
 	$(TONE_MAP_TEST_TARGET)
+	$(SENSOR_BLOOM_TEST_TARGET)
 	python3 tests/test_camera_cli.py $(BUILD_DIR) $(TEST_OUT_DIR)
 
 tone-map-test: $(TONE_MAP_TEST_TARGET)
 	$(TONE_MAP_TEST_TARGET)
 
+sensor-bloom-test: $(SENSOR_BLOOM_TEST_TARGET)
+	$(SENSOR_BLOOM_TEST_TARGET)
+
+sensor-bloom-bench: $(SENSOR_BLOOM_BENCH_TARGET)
+
 fast-psf-fftw-bench: $(FAST_PSF_FFTW_BENCH_TARGET)
 
 clean: