Codec roadmap: define envelope specs + validation tests

[bbopen]

Goal

Define envelope specs and validation tests for SciPy, Torch, and Sklearn codecs per docs/codec-roadmap.md.

Scope

• Specify envelope fields and encoding options for SciPy/Torch/Sklearn
• Decide on versioning (codecVersion?) and size limits
• Add validation + round-trip tests (including size limit/fallback cases)

Acceptance criteria

• Specs documented in docs/codec-roadmap.md (or a new codec spec doc)
• Validation tests exist for each envelope type
• Size limits and fallback behavior are explicit and tested

[bbopen]

Resolved by #35 (envelope specs + validation tests + size cap).