Add Java and Python memory stores with composable decorators - #3

Closes #2.

Summary

• Add a minimal Java 17 MemoryStore contract with RAM and filesystem implementations.
• Add a delegating decorator and RAM read-through/write-through cache decorator.
• Add a Python 3.10+ implementation with matching RAM, filesystem, and decorator behavior.
• Add contract tests for RAM/filesystem backends and decorator tests in both languages.
• Document the storage contract, filesystem assumptions, cache coherence, lifecycle ownership, and cross-language parity in README and ADR-0002.
• Add GitHub Actions CI for Maven tests and Python unittest discovery.

Design notes

• Filesystem IDs are hex-encoded, never interpreted as paths.
• Filesystem writes use temporary files and atomic replacement where supported.
• No framework, ORM, database, or cache library is introduced.
• Filesystem stores are intended for single-process use; independent-process coordination is out of scope.
• The outermost decorator owns and closes the wrapped store.
• This is a storage foundation, not the remote REST service contract or retrieval/ranking implementation.

Validation

Test suites and CI workflow are included. They have not yet been confirmed green, so keep this PR in draft until CI executes successfully.