prisma-risk / tsoracle / 26425430778

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feat(codec): add VersionedCodec trait and framed encode/decode helpers (#454)

Add a symmetric multi-version codec seam to tsoracle-codec so types whose
on-disk and on-wire layout evolves can be read from any supported version
(up-convert) and written at any active version (down-convert), with
fail-loud rejection of out-of-range versions and fail-loud rejection of
non-representable down-conversions.

- New `versioned` module exposes `VersionedCodec` (`decode_version` /
  `encode_version`), `encode_framed` / `decode_framed` (range-checked
  version-prefix framing), and `encode_postcard` / `decode_postcard_exact`
  (DRY per-version body building blocks implementors call).
- New `CodecError` variants `VersionUnsupported { min, max, actual }`
  (foreign-version range rejection on read) and `NotRepresentable { version }`
  (encode-side feature-gate violation).
- `codec_io_error` extended so `VersionUnsupported` also maps to
  `io::ErrorKind::InvalidData`, preserving the foreign-format storage
  contract for the openraft storage layer once it adopts `decode_framed`.
  `NotRepresentable` maps to `Other`, not `InvalidData`.

Purely additive: the existing single-version `encode` / `decode` and
their callers are untouched. No production call site changes.

Tests: per-version body round-trip, error variant Display / io mapping,
trait round-trip across two versions, fail-loud negative test for an
unrepresentable down-conversion, framed encode/decode (in-range
dispatch, out-of-range / below-range / empty rejection), and a proptest
covering framed round-trip at both versions.

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