fix(ha-light): apply brightness_scale once and respect boost multipliers

`_send_entity_color` was multiplying the per-mapping `brightness_scale`
into the brightness payload twice when the effective scale was below 1,
yielding a quartered output for a configured half-scale. Conversely,
when the value-stream multiplier exceeded 1.0 with a default scale,
the entire scaling step was skipped and the boost was lost.

Compute brightness as `clamp(max(r,g,b) * bs * vs, 0, 255)` once and
ship it directly, with regression tests pinning the half-scale, boost,
and 255-clamp cases.

server/tests/core/test_ha_light_target_processor.py

@@ -234,6 +234,112 @@ async def test_css_mode_still_uses_per_segment_average():
     assert css_mgr.released == [("css_1", "t_css")]
 
 
+@pytest.mark.asyncio
+async def test_brightness_scale_applied_exactly_once():
+    """Regression: brightness_scale must not be double-applied.
+
+    Past bug: brightness was first multiplied by (bs * vs_multiplier) when
+    eff_scale < 1.0, then multiplied by `bs` again before being sent. With
+    bs=0.5 and vs=1.0 the user got quarter brightness instead of half.
+    """
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+    color_stream = _FakeColorStream((200, 100, 50))
+
+    proc = HALightTargetProcessor(
+        target_id="t_b",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_c",
+        light_mappings=[_mapping("light.a", scale=0.5)],
+        color_tolerance=0,
+        ctx=_make_ctx(ha_manager=ha_mgr, vs_manager=_FakeVSManager(color_stream)),
+    )
+
+    await proc.start()
+    for _ in range(40):
+        await asyncio.sleep(0.05)
+        if any(c.target["entity_id"] == "light.a" for c in runtime.calls):
+            break
+    await proc.stop()
+
+    turn_on = next(c for c in runtime.calls if c.service == "turn_on")
+    # max(200,100,50)=200, scaled by bs=0.5 → 100. Pre-fix bug produced 50.
+    assert turn_on.service_data["brightness"] == 100
+    assert turn_on.service_data["rgb_color"] == [200, 100, 50]
+
+
+@pytest.mark.asyncio
+async def test_brightness_boost_via_value_stream_multiplier():
+    """vs_multiplier > 1.0 must boost brightness up to the 255 cap.
+
+    Past bug: scaling was gated behind `if eff_scale < 1.0`, so any boost
+    direction (vs > 1.0 with bs <= 1.0) was silently ignored.
+    """
+
+    class _BoostingStream:
+        def get_value(self) -> float:
+            return 2.0  # 2× boost
+
+        def get_color(self):
+            return (50, 30, 10)
+
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+    boosting_stream = _BoostingStream()
+
+    proc = HALightTargetProcessor(
+        target_id="t_boost",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_color",
+        # Brightness VS is a separate stream — wire it through bfloat source_id.
+        brightness=BindableFloat(1.0, source_id="vs_brightness"),
+        light_mappings=[_mapping("light.a", scale=1.0)],
+        color_tolerance=0,
+        ctx=_make_ctx(ha_manager=ha_mgr, vs_manager=_FakeVSManager(boosting_stream)),
+    )
+
+    await proc.start()
+    for _ in range(40):
+        await asyncio.sleep(0.05)
+        if any(c.target["entity_id"] == "light.a" for c in runtime.calls):
+            break
+    await proc.stop()
+
+    turn_on = next(c for c in runtime.calls if c.service == "turn_on")
+    # max(50,30,10)=50, vs_multiplier=2.0 → 100. Pre-fix bug produced 50.
+    assert turn_on.service_data["brightness"] == 100
+
+
+@pytest.mark.asyncio
+async def test_brightness_clamped_at_255():
+    color_stream = _FakeColorStream((200, 0, 0))
+    runtime = _FakeHARuntime()
+    ha_mgr = _FakeHAManager(runtime)
+
+    proc = HALightTargetProcessor(
+        target_id="t_clamp",
+        ha_source_id="ha_1",
+        source_kind="color_vs",
+        color_value_source_id="vs_c",
+        # bs=4.0 with channel max 200 → 800, must be clamped to 255.
+        light_mappings=[_mapping("light.a", scale=4.0)],
+        color_tolerance=0,
+        ctx=_make_ctx(ha_manager=ha_mgr, vs_manager=_FakeVSManager(color_stream)),
+    )
+
+    await proc.start()
+    for _ in range(40):
+        await asyncio.sleep(0.05)
+        if any(c.target["entity_id"] == "light.a" for c in runtime.calls):
+            break
+    await proc.stop()
+
+    turn_on = next(c for c in runtime.calls if c.service == "turn_on")
+    assert turn_on.service_data["brightness"] == 255
+
+
 @pytest.mark.asyncio
 async def test_get_state_reports_source_kind():
     runtime = _FakeHARuntime()