Function resolveSpecVersion

The TDF spec version a manifest records, or undefined if it records none.

Precedence is schemaVersion, then tdf_spec_version at the root, then tdf_spec_version under payload. Only non-empty strings count; any other value (the key is known in the wild carrying null) is skipped rather than treated as an error, since reporting malformed manifests is not this function's job.

schemaVersion is the canonical name. tdf_spec_version is not a former spelling that was renamed; it entered some specification drafts and older documentation in error, and writers built from them emitted it. Both of its placements are probed because both occur in archival files. The root is where the spec's own manifest.md has always documented the field, and where this SDK wrote it (until #414 switched the writer to schemaVersion) and has read it since. Under payload is where revisions of the JSON schema declared it in error, which led at least one writer to emit the key there with a null value. Nothing is ever written under the non-aligned name: the writer emits schemaVersion only.

The reader uses the result to choose how the integrity digests are encoded: hex before 4.3.0, raw bytes since. A manifest that records no version is read as 4.2.2.

Mirrors the Go SDK's Manifest.UnmarshalJSON (opentdf/platform#4060).