fix(go): don't emit embeds for interface type-set constraints

[Synvoya]

Problem

The Go extractor treats every interface type_elem as an embedded interface.
But a Go type-set constraint (generics) is not embedding — a union
A | B or an approximation ~T is a set of type terms, and Go only ever
embeds a lone interface type. Because the branch walked union terms the same
way it walks an embedded interface, each non-predeclared term produced a
spurious embeds heritage edge.

Reproduction on v8 HEAD

package p
type MyInt int
type MyFloat float64
type Number interface {
    MyInt | MyFloat
}

extract_go emits:

Number --embeds--> MyInt
Number --embeds--> MyFloat

embeds is a heritage relation (grouped with inherits / implements /
mixes_in in extract.py), so this asserts a composition/method-promotion
relationship that does not exist — Number cannot embed the concrete types
MyInt/MyFloat; they are constraint terms. This pollutes heritage queries
over the graph with false edges.

Fix

Detect a type-set type_elem (it has a | or a negated_type child — both
tokens appear only in constraint position, never in embedding) and reclassify
its terms as references (context="type_constraint") instead of embeds.
This keeps the type link but drops the false heritage claim. The edge count is
unchanged; only the relation/context is corrected.

Genuine interface embedding is untouched — a lone interface term
(Reader, io.Reader, Sortable[int]) has no |/~, so it still emits
embeds.

Tests

Added to tests/test_languages.py:

• test_go_interface_type_union_is_not_embedding — negative control:
  union terms must not be embeds, and are present as
  references/type_constraint. Fails on HEAD (asserts the spurious
  Number->MyInt embed away), passes with the fix.
• test_go_interface_embedding_emits_embeds /
  test_go_struct_embedding_emits_embeds — positive guards that genuine
  embedding on the existing sample.go fixture (ReaderLogger embeds
  Logger/Reader; DataProcessor embeds BaseProcessor) still works.

python3 -m pytest tests/test_languages.py -q → 317 passed, 13 skipped
(the 13 skips are the optional tree-sitter-dm grammar). No behavior change
outside Go interface type-set constraints.