Make mlquant demo portable outside a source checkout

[initial-d]

Summary

• package the deterministic small demo config inside mlquant
• resolve that resource when mlquant demo is run without --config
• preserve explicit --config PATH behavior
• add regression coverage for running from an isolated working directory
• keep the packaged and repository configs synchronized with a test

Why

The current default points at configs/small.yaml, which only exists inside a repository checkout. A wheel or future PyPI install can expose the mlquant command but cannot complete the advertised zero-setup demo from another directory. This change removes that first-run packaging trap.

Validation

• Python bytecode compilation passed
• repository and packaged YAML files compare byte-for-byte
• built mlquant-0.2.4-py3-none-any.whl locally
• inspected the wheel and confirmed mlquant/configs/small.yaml is included
• full lint, tests, and CLI smoke test run in GitHub CI