Add enclosing ranges for Go declarations

[jordigilh]

Closes #297.

Emit typed enclosing ranges for type, var/const, and struct-field definitions, including attached docs, and declare UTF-8 byte positions. Existing function ranges and primary occurrence sites remain unchanged.

Validation: go test ./...; on the frozen Go fixture, valid definition enclosures increased 27/150 → 58/150 with all 471 primary sites unchanged. A separate Code IR shadow recheck joined 117/117 selected-local candidates.

[jordigilh]

After addressing the comment on the Interface I did a follow up analysis on other gaps and I identified 3 more areas that could be covered in this PR if you're ok with it:

• Generic type parameter definitions, such as T in type Box[T any], aren’t given their own enclosing ranges.
• Block-local declarations (type, const, and var), and fields in anonymous struct types inside function signatures or bodies, aren’t collected; the current prepass scans file-level declarations.
• For grouped declarations, individual specs get ranges, but a doc comment attached to the group’s GenDecl is omitted when there are multiple specs. Should that shared comment be included in each spec’s range?

What do you think? Should I go ahead and extend the PR scope to cover for these use cases?

[jupblb]

Thanks for the contribution! LGTM :)

What do you think? Should I go ahead and extend the PR scope to cover for these use cases?

These are all good observations but I'd prefer these changes are made in another PR, if that's not too much to ask.