Remove unnecessary ProGuard keep rule and extract SaveResult to own file

The -keep rule for no.naiv.tiltshift.effect.** was based on the
incorrect assumption that GLSL shaders use Java reflection. Shader
source is loaded from raw resources — all effect classes are reached
through normal Kotlin code and R8 can trace them. Removing the rule
lets R8 properly optimize the effect package.

Also extract SaveResult sealed class from PhotoSaver.kt into its own
file to match the documented architecture.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/proguard-rules.pro

@@ -1,5 +1,2 @@
 # Add project specific ProGuard rules here.
 # CameraX and GMS Location ship their own consumer ProGuard rules.
-
-# Keep OpenGL shader-related code (accessed via reflection by GLSL pipeline)
--keep class no.naiv.tiltshift.effect.** { *; }