Add UTF-8 Myers diff with JSON serialization

str_diff_patch2.h runs Myers diff on UTF-8 codepoints (via UTF-32), maps edits back to byte-offset replace tuples, validates UTF-8 inputs, and serializes diffs as JSON arrays with one entry per line.

Tests: str_diff_patch2_test.cpp covers ASCII/UTF-8 edits and JSON round trips; test/physwiki_diff_check.cpp batches adjacent diffs for groups of *_.tex files, writes JSON diff files, and verifies reconstruction from the first to last file.

Utilities: test/physwiki_backup_db.cpp builds PhysWiki-backup.db from PhysWiki-backup.sql using SQLiteCpp and loads all YYYYMMDDHHMM_AuthorID_ArticleId.tex files.

CMake: added test targets in CMakeLists.txt, SLISC/str/CMakeLists.txt, and test/CMakeLists.txt.