Go member calls resolve to nothing across files, and cannot cross a repo boundary at all

[xiongjianxu]

What happens

A Go method call on a receiver resolves only when the bare method name happens to
match a symbol declared in the caller's own file. Anything else produces no edge:

// svc/greeter.go
package svc

type Greeter struct{}

func (g *Greeter) Greet() {}

// svc/app.go
package svc

type App struct {
	greeter *Greeter
}

func (a *App) Run() { a.greeter.Greet() }   // no `calls` edge

extract_go reads the receiver's identifier and discards it unless it names an
imported package, so the shape Go dependency injection actually takes — a struct
field used from a method — contributes nothing to the call graph. The shared
cross-file pass then skips member callees outright (obj.log() → "log" has no
import evidence), and no _resolve_go_* resolver is registered to pick them up.
No test in the repo asserts that a cross-file Go receiver method call yields an
edge; test_go_builtin_call_targets.py states the omission explicitly.

Merging two repos loses the same edges for the same reason, plus one more: Go
nodes never carry the _callable / _callable_class markers (#2438), because
extract_go is the one extractor outside the tree-sitter engine and the engine is
the only place that stamps them. link_cross_repo_member_calls requires
_callable_class on the declaring type, so a Go type could not answer a parked
call even if one existed.

What it should do

Bind the call through the receiver's declared type, as the Swift (#1356), C++
(#1547), ObjC (#1556) and C# (#1609) resolvers already do for their languages —
and when the type is declared nowhere in the build, park the call so a merged
graph can finish it (#3152).

Go writes the receiver's type down in four places, all of them cheap to read:

shape	source
type App struct { greeter *Greeter }	struct field
func (s *Server) M() / func f(g *Greeter)	parameter
var g Greeter	var declaration
g := Greeter{} / g := &Greeter{}	composite literal

The single-definition guard has to stay: a Go type node id folds in the package
directory, so two packages declaring Greeter are two nodes and without import
evidence neither one is the answer.

Why it matters

Go is a backend language and services are split across repositories; the call
graph across that boundary is the reason to merge two graphs at all. Together with
the ObjC (#3384), TS/JS (#3386), Kotlin (#3388) and PHP (#3390) gaps, this covers
the languages a mobile-plus-backend org actually ships.

PR to follow.