chore: remove obsolete temp files, audit reports, progress summaries, and non-English docs

Removed 29 files:

- 11 root temp/scratch files (tmp_*, temp_*, .tmp_*, edit_sln.ps1)

- 8 obsolete progress reports and audit docs

- 9 completion announcements and version summaries

- 1 non-English doc (REGISTRATION.nl.md)

- Updated .gitignore to prevent future tmp_/temp_/.tmp_ tracking

Author: MPCoreDeveloper

.gitignore

@@ -385,6 +385,9 @@ BenchmarkDotNet.Artifacts/
 # Test databases
 test_*/
 temp_*/
+temp_*
+tmp_*
+.tmp_*
 
 # PowerShell scripts (temporary restructuring scripts)
 restructure-sharpcoredb.ps1