feat(cv): add CVTuning struct and NVS persistence scaffolding

Adds CVTuning to CVState, populates defaults from existing file-scope
constants in cv_init, and introduces config_load_tuning/config_save_tuning
backed by the doorcounter NVS namespace. No runtime behavior change yet;
CV code still reads the existing constants (Task 2 will migrate reads).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

firmware/src/config.h

@@ -1,6 +1,7 @@
 // firmware/src/config.h
 #pragma once
 #include <Arduino.h>
+#include "cv.h"
 
 struct DeviceConfig {
     String device_id;      // e.g. "dc-0042"
@@ -22,3 +23,10 @@ bool config_has_wifi();
 
 // Erase WiFi credentials only (factory reset — preserves device_id etc).
 void config_clear_wifi();
+
+// Load CV tuning from NVS. Returns true only if all keys present (cfg_version sentinel).
+// If any key missing, tuning is NOT modified (caller keeps its defaults).
+bool config_load_tuning(CVTuning& tuning);
+
+// Save CV tuning to NVS atomically. Returns true if all writes succeeded.
+bool config_save_tuning(const CVTuning& tuning);