Add multi-language codec support via Docker

[jakedgy]

Summary

Adds support for non-Rust codec submissions via Docker containers:

• New --docker CLI flag enables external codec discovery and execution
• Auto-discovers codecs by scanning src/*/Dockerfile
• Implements stdin/stdout ABI for encode/decode operations
• Auto-enables Docker mode when targeting an external codec (e.g., --codec example-python)
• Includes example Python codec demonstrating the ABI
• Updates CI workflow to detect and test Docker-based submissions

Resolves #3

How This Was Built

This PR was developed as an educational experiment using Claude Code with the superpowers plugin. The workflow:

1. Brainstorming - Collaborative design session to define the ABI, discovery mechanism, and CLI interface
2. Planning - Generated a detailed 11-task implementation plan
3. Subagent-driven development - Each task implemented by a fresh subagent with spec compliance review

The entire conversation and development happened in a single Claude Code session. This is experimental - feedback welcome!

Test Plan

• cargo run --release -- --codec example-python shows example-python results
• cargo run --release -- --codec naive works without Docker (existing behavior preserved)
• Round-trip verification passes (encode → decode → compare)
• cargo fmt --check passes

Example Output

┌────────────────────────┬────────────────┬────────────┐
│ Codec                  │           Size │ vs Naive   │
├────────────────────────┼────────────────┼────────────┤
│ Naive                  │    210,727,389 │   baseline │
│ example-python         │     25,837,747 │     -87.7% │
└────────────────────────┴────────────────┴────────────┘

All verifications passed

🤖 Generated with Claude Code + superpowers plugin

[agavra]

Thanks @jakedgy - since this is a pretty big change I want to have time to review it properly. I probably won't get to it until next week since I'm out this weekend.