store: memory + sqlite backends + --store scheme

[fruwe]

Closes #22.

Implements the memory and SQLite store backends and the --store scheme (SPEC §8/§13.1).

store.py refactor to a small Store interface (abstract load/save) with three
backends behind it:

• FileStore — the existing JSON-files-in-a-dir store (file:<dir> / bare <dir>).
• MemoryStore — in-process, ephemeral (mem:); holds the state in memory, isolated per
  process (a run session, §13.7).
• SqliteStore — single-file persistent DB (sqlite:<path>); sqlite3 is stdlib, no new
  dependency.
• open_store(spec) parses the scheme (file: / mem: / sqlite:, bare = file).

CLI: --store now selects a backend via open_store; the default is unchanged
(./.harel, file). All backends are behaviorally identical — same CLI results and the same
§8 snapshot JSON (the on-disk/in-memory layout is an implementation detail).

Tests: round-trip an instance through each backend (parity of snapshot JSON); a sqlite:
store persists across CLI invocations; a mem: store is isolated per process but holds state
within one run session; file and sqlite produce identical CLI results. ruff + mypy
clean; 164 tests pass; the cli/03-stepping black-box case still passes.