[ci] Add lint check to prevent powf in core and base entity platforms

Add a ci-custom.py check that prevents powf() from being introduced
into esphome/core/ and base entity platform components (sensor, light,
climate, etc.). These files are linked into every build, and powf
pulls in __ieee754_powf (~2.3KB flash).

Existing legitimate uses of powf with non-integer exponents (gamma
correction in helpers.cpp) are excluded. New uses can opt out with
// NOLINT if truly necessary.

esphome/core/helpers.cpp

@@ -714,7 +714,7 @@ float gamma_correct(float value, float gamma) {
   if (gamma <= 0.0f)
     return value;
 
-  return powf(value, gamma);
+  return powf(value, gamma);  // NOLINT - deprecated, removal 2026.9.0
 }
 float gamma_uncorrect(float value, float gamma) {
   if (value <= 0.0f)
@@ -722,7 +722,7 @@ float gamma_uncorrect(float value, float gamma) {
   if (gamma <= 0.0f)
     return value;
 
-  return powf(value, 1 / gamma);
+  return powf(value, 1 / gamma);  // NOLINT - deprecated, removal 2026.9.0
 }
 
 void rgb_to_hsv(float red, float green, float blue, int &hue, float &saturation, float &value) {

esphome/core/helpers.h

@@ -440,7 +440,7 @@ template<size_t STACK_SIZE, typename T = uint8_t> class SmallBufferWithHeapFallb
 ///@{
 
 /// Compute 10^exp using iterative multiplication/division.
-/// Avoids pulling in powf/__ieee754_powf (~2.3KB flash) for small integer exponents.
+/// Avoids pulling in powf/__ieee754_powf (~2.3KB flash) for small integer exponents.  // NOLINT
 /// Exact for non-negative exponents up to 10 (powers of 10 up to 10^10 are exact in float).
 inline float pow10_int(int8_t exp) {
   float result = 1.0f;

script/ci-custom.py

@@ -825,6 +825,54 @@ def lint_no_scanf(fname, match):
     )
 
 
+# Base entity platforms - these are linked into most builds and should not
+# pull in powf/__ieee754_powf (~2.3KB flash).
+BASE_ENTITY_PLATFORMS = [
+    "alarm_control_panel",
+    "binary_sensor",
+    "button",
+    "climate",
+    "cover",
+    "datetime",
+    "event",
+    "fan",
+    "light",
+    "lock",
+    "media_player",
+    "number",
+    "select",
+    "sensor",
+    "switch",
+    "text",
+    "text_sensor",
+    "update",
+    "valve",
+    "water_heater",
+]
+
+# Directories protected from powf: core + all base entity platforms
+POWF_PROTECTED_DIRS = ["esphome/core"] + [
+    f"esphome/components/{p}" for p in BASE_ENTITY_PLATFORMS
+]
+
+
+@lint_re_check(
+    r"[^\w]powf\s*\(" + CPP_RE_EOL,
+    include=[
+        f"{d}/*.{ext}" for d in POWF_PROTECTED_DIRS for ext in ["h", "cpp", "tcc"]
+    ],
+)
+def lint_no_powf_in_core(fname, match):
+    return (
+        f"{highlight('powf()')} pulls in __ieee754_powf (~2.3KB flash) and is not allowed in "
+        f"core or base entity platform code. These files are linked into every build.\n"
+        f"Please use alternatives:\n"
+        f"  - {highlight('pow10_int(exp)')} for integer powers of 10 (from helpers.h)\n"
+        f"  - Precomputed lookup tables for gamma/non-integer exponents\n"
+        f"(If powf is strictly necessary, add `// NOLINT` to the line)"
+    )
+
+
 @lint_content_find_check(
     "ESP_LOG",
     include=["*.h", "*.tcc"],