Codec roadmap: JS decoder + type mapping presets

[bbopen]

Goal

Implement JS decoder support and type mapping presets for new codec envelopes (SciPy/Torch/Sklearn), per docs/codec-roadmap.md.

Scope

• Extend src/utils/codec.ts for new envelope fields/encodings.
• Add type mapping presets in src/types as needed.
• Make Arrow DX frictionless: auto-register decoder when apache-arrow is installed, or provide a tiny helper consumed by generated TS/runtime bootstrap.
• Ensure cross-runtime behavior (Node/Bun/Deno/Browser) where feasible.
• Update docs and living app to exercise the Arrow path by default.

Acceptance criteria

• Decoder handles all envelope variants in the spec.
• Type mappings are exposed and documented.
• Arrow decoding requires no manual wiring when apache-arrow is present.
• Tests cover decoding across supported runtimes (where feasible).

References

• docs/codec-roadmap.md

[bbopen]

Roadmap updated in docs/codec-roadmap.md. JS decoder work should include Arrow DX (auto-register decoder when apache-arrow is present) plus presets/tests. This issue stays as the decoder + mapping implementation track after #24.

[bbopen]

Resolved by #35 and #37 (decoder support + presets + Arrow auto-register + docs/tests).