build: fix desktop texture regeneration crash on clean tree

stale-check compared LastWriteTime against a stamp file that does not
exist yet on the first build, so Get-Item threw PathNotFound and aborted
the build before make DSK.

- build.ps1: guard the texSrc/texStamp mtime comparison with
  Test-Path $TexStamp; $needTex is already $true when the stamp is
  absent, so the comparison is only meaningful when both files exist
- .gitignore: ignore IDEA.md local scratch note

Verified: .\build.bat now reaches gen_textures.py and emits
back.rgba / startMenuLogo.rgba / bootlogo.rgba into
SYSTEM/system/deshab64/desktop/textures/.

.gitignore

@@ -16,7 +16,7 @@ target/
 
 *.dbf
 *.rgba
-
+IDEA.md
 SYSTEM/boot/linux-extra-rootfs.img
 *.d
 target/

build.ps1

@@ -766,7 +766,7 @@ if (Test-Path $GenTex) {
     $TexSrc   = Join-Path $Root 'SYSTEM\system\deshab64\desktop\textures\startMenuLogo.png'
     $TexStamp = Join-Path $Root 'SYSTEM\system\deshab64\desktop\textures\startMenuLogo.rgba'
     $needTex = -not (Test-Path $TexStamp)
-    if ($needTex -and (Test-Path $TexSrc)) {
+    if ($needTex -and (Test-Path $TexSrc) -and (Test-Path $TexStamp)) {
         $needTex = (Get-Item $TexSrc).LastWriteTime -gt (Get-Item $TexStamp).LastWriteTime
     }
     if ($needTex) {