Minor: Clarify value_metadata in Variant value layout diagram

[douenergy]

Rationale for this change

When first reading the Variant encoding spec, it can be unclear that each Variant value begins with a single value_metadata byte composed of value_header and basic_type. This can be confusing, especially since “metadata” is also used to refer to the dictionary metadata structure.

The spec already uses the term value_metadata in prose, so labeling it clearly in the layout diagram helps reinforce understanding and reduce ambiguity.

What changes are included in this PR?

• Added a clarifying comment (<-- value_metadata) to the value layout diagram to explicitly indicate that the first byte is value_metadata.

Do these changes have PoC implementations?

N/A — documentation-only change, no impact on implementations.

[alamb]

Thank you @douenergy -- I think this is a nice clarification (note I don't have permissions to merge to this repo so we will need to wait for a committer to approve as well)

[wgtmac]

Looks good! Thanks @douenergy and @alamb!

[alamb]

Thank you for the merge @wgtmac