Centralize build output and generate OS-specific startup.json

- All DLLs now output to lib\[Configuration]\[TargetFramework]\ at repo root (see Directory.Build.props)
- .gitignore updated to exclude /lib/
- On first run, startup.json is auto-generated with OS-appropriate default paths (Windows, Linux, macOS, or portable)
- Removes null/example config; generated config is immediately usable and clearly documented
- Extensive new documentation: build output, startup.json logic, visual guides, and code proofs
- Publish profile now deletes existing files for clean deploys
- No breaking changes: existing startup.json files are preserved
- Improves first-run UX, deployment, and cross-platform consistency

.gitignore

@@ -1,4 +1,4 @@
-################################################################################
+################################################################################
 # This .gitignore file was automatically created by Microsoft(R) Visual Studio.
 ################################################################################
 
@@ -19,4 +19,8 @@
 *.dll
 *.pdb
 bin/
-obj/
+obj/
+
+# Centralized lib output folder
+/lib/
+lib/