build: preserve F303 float constant semantics

Makefile

@@ -13,7 +13,9 @@ endif
 # Compiler options here.
 ifeq ($(USE_OPT),)
  ifeq ($(TARGET),F303)
-USE_OPT = -Og -fno-inline-small-functions -ggdb -fomit-frame-pointer -falign-functions=16 --specs=nano.specs -fstack-usage -std=c11
+USE_OPT = -Og -fno-inline-small-functions -ggdb -fomit-frame-pointer -falign-functions=16 --specs=nano.specs -fstack-usage -fsingle-precision-constant -std=c11
+# The legacy ChibiOS rules appended -fsingle-precision-constant for hard-FPU
+# builds. Keep that numeric behavior explicit when using the 21.11.x rules.
 #USE_OPT+=-fstack-protector-strong
  else
 USE_OPT = -Og -fno-inline-small-functions -ggdb -fomit-frame-pointer -falign-functions=16 --specs=nano.specs -fstack-usage -fsingle-precision-constant 

docs/CHIBIOS_21_11_5_PORT.md

@@ -28,7 +28,12 @@ this port is marked ready.
   queue reset calls, time conversions, and thread diagnostics.
 - Update the project-local F303 ADC LLD while retaining its tinySA-specific
   behavior.
-- Convert custom `BaseSequentialStream` VMTs to the current layout.
+- Convert custom `BaseSequentialStream` VMTs to the current layout, including
+  the required `instance_offset` field.
+- Preserve the legacy hard-FPU build's `-fsingle-precision-constant` behavior
+  explicitly. ChibiOS 21.11.x no longer adds it automatically; omitting it
+  promotes unsuffixed sweep constants to software double precision and causes
+  a measurable self-test sweep regression.
 
 ## Build