fix(go-parser): key generic-type methods on base type, not "unknown"

[gadievron]

Summary

The Go parser keys methods on generic types (func (s *Stack[T]) Push(...), func (p *Pair[K,V]) Key(...)) under a bogus class name unknown instead of the real type, because the receiver-type stringifier typeToString has no case for Go's generic-instantiation AST nodes (*ast.IndexExpr / *ast.IndexListExpr) and falls through to default: return "unknown".

Impact (all confirmed on this branch's base):

• Wrong unit id — m.go:unknown.Push instead of m.go:Stack.Push.
• Silent data loss — distinct generic types' same-named methods (Stack[T].Len, Queue[T].Len) collapse to one id m.go:unknown.Len; the store (output.Functions[id] = info) overwrites, so one real unit is dropped.
• Dropped call edges — generic call sites Gen[K,V]() (*ast.IndexListExpr) and generic method calls obj.M[T]() were not recovered by analyzeCallExpr.

Generic types are idiomatic Go since 1.18, so this hits modern codebases broadly.

Changes

• extractor.go typeToString: add case *ast.IndexExpr and case *ast.IndexListExpr, each recursing into t.X so the class key is the bare base type (Stack, Pair).
• callgraph.go analyzeCallExpr: unwrap a leading IndexExpr/IndexListExpr then reuse the existing Ident/SelectorExpr classification — fixes all four generic call shapes and recovers the receiver for generic method calls.
• extractor.go duplicateIDWarning: emit an os.Stderr warning when a funcID collides on store, so any future class-key collapse fails loudly instead of silently overwriting. JSON output shape unchanged.

Tests

5 new Go tests: generic-receiver class keys (pointer/value/multi-param), the no-collision data-loss regression (Stack.Len + Queue.Len stay distinct), generic call recovery, and the dup-guard. go test ./... green; gofmt/go vet clean.

Scope & related work (please read before requesting "why not Zig/PHP too?")

This PR is Go-only by design and is self-contained — it touches only parsers/go/go_parser/ and has no cross-parser dependency. The Go parser uses the standard-library go/ast, whose node types are stable, so this lands cleanly on master.

The same class-key-collapse family also affects the Zig and PHP parsers. Those fixes are in a separate PR that depends on the parser-rewrite PRs #110 (Zig) and #111 (PHP) and so targets the staging/parser-fix-stack integration branch, not master. This Go PR is independent of all of that — go/ast node types are stable — so do not block it on them.

Notes

• For the class key the [T] type arguments are intentionally dropped (we want Stack, not Stack[T]); the receiver string is a display field, not used for resolution.
• Known pre-existing, out-of-scope edges (not introduced here): a slice/map-index call funcs[i]() reports the container identifier as the call name; a parenthesized receiver func ((T)) M() still hits default (now observable via the dup-guard).