preserve flash / full erase flash

- preserve: bonding 정보 및 settings/NVS 유지, application만 다시 올리는 용도

tools/flash_full_erase.ps1

@@ -0,0 +1,19 @@
+param(
+    [string]$Hex = "build\merged.hex"
+)
+
+Set-StrictMode -Version Latest
+$ErrorActionPreference = "Stop"
+
+$RepoRoot = Split-Path -Parent $PSScriptRoot
+$HexPath = Join-Path $RepoRoot $Hex
+
+if (-not (Test-Path -LiteralPath $HexPath)) {
+    throw "Hex file not found: $HexPath. Build the project first."
+}
+
+Write-Host "[FLASH] FULL ERASE mode"
+Write-Host "[FLASH] This wipes application flash, NVS/settings, and BLE bonds."
+Write-Host "[FLASH] Programming: $HexPath"
+
+nrfjprog --program $HexPath --chiperase --verify --reset