Add initial SQLmem package structure with SQL parser, cache manager, column registry, and tests

CHANGELOG.md

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+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [Unreleased]
+
+### Added
+- Project specification in `project.md` — architecture, API design, cache backend, metadata schema, logging strategy, and TODO for future features (JOIN, SELECT * support)
+- `.gitignore` for Python/Poetry project
+- `pyproject.toml` dependencies: `sqlglot`, `sqlalchemy`, `loguru`, `python-dotenv`; dev dependencies: `pytest`, `ruff`, `mypy`
+- `src/sqlmem/` package structure with src layout
+- `src/sqlmem/exceptions.py` — `ReadOnlyError` (blocks INSERT/UPDATE/DELETE), `UnsupportedQueryError` (blocks JOIN and SELECT *)
+- `src/sqlmem/config.py` — loads `.env`, configures `loguru` with DEBUG/INFO level based on `SQLMEM_DEBUG`
+- `src/sqlmem/_meta.py` — package version constant
+- `src/sqlmem/parser.py` — SQL Parser using `sqlglot`; extracts table and columns from SELECT, raises on writes/JOIN/wildcard
+- `src/sqlmem/registry.py` — Column Registry; accumulates requested columns per table, detects missing columns requiring re-fetch
+- `src/sqlmem/cache.py` — Cache Manager; SQLite in-memory storage, load from `cache.db` on startup (with schema version check), hourly backup thread, `atexit`/SIGTERM flush, metadata tables (`_sqlmem_meta`, `_sqlmem_tables`, `_sqlmem_columns`)
+- `src/sqlmem/executor.py` — Query Executor; cache hit/miss logic, re-fetch on new columns with WARNING log
+- `src/sqlmem/engine.py` — `CachingEngine` wrapper; public API compatible with SQLAlchemy, `invalidate(table)` for manual cache clearing
+- `src/sqlmem/__init__.py` — public exports: `CachingEngine`, `ReadOnlyError`, `UnsupportedQueryError`
+- `tests/test_parser.py` — parser tests: SELECT parsing, ReadOnlyError, UnsupportedQueryError
+- `tests/test_cache.py` — cache tests: load, data correctness, metadata, disk backup/reload
+- `tests/test_registry.py` — registry tests: accumulation, needs_refetch, table isolation