Convert mewe and walStrings to LAVA @artifact_processor

[abrignoni]

Converts two more parsers to the LAVA @artifact_processor pattern.

• mewe — split the single old function into two decorated artifacts: MeWe - Chat (CHAT_MESSAGE join; createdAt raw → aware UTC datetime, replacing SQL DATETIME(...,'unixepoch')) and MeWe - SGSession (Key/Value from SGSession.xml). Each now has its own path so it only opens the file it needs.
• walStrings — extracts ASCII strings from SQLite WAL/journal files into per-file .txt reports under the report folder and links them; Report declared in html_columns; added encoding to both open() calls. (Writes only into the report output folder, never the source.)

Verified: byte-compile, decorated 4-arg signatures, return 3-tuples, output_types valid, paths are tuples, loader resolves all functions, pylint clean.