Summary

• Add a UTF-8 reader that warns and replaces invalid byte sequences.
• Use it for indexing, expand, and maintenance journal reads.
• Add regression coverage for index, expand, and maintenance paths.

Test Plan

• .venv/bin/python -m pytest tests/test_core_encoding.py tests/test_cli_error_handling.py tests/test_maintenance.py -q
• .venv/bin/python -m pytest tests/test_embed_batching.py tests/test_scanner.py -q
• .venv/bin/python -m py_compile src/memsearch/io.py src/memsearch/core.py src/memsearch/cli.py src/memsearch/maintenance.py tests/test_core_encoding.py tests/test_cli_error_handling.py tests/test_maintenance.py

Note: full pytest was attempted in a temporary pip-built environment, but that environment installed pymilvus/milvus-lite 3.0 instead of uv.lock versions and failed tests/test_store.py::test_collection_description due to dependency behavior. uv sync --locked --dev was attempted but failed downloading numpy due to network timeout.