fix: make the native libpostal backend opt-in

[immanuwell]

Native libpostal was getting picked whenever cgo was on. On a pretty normal Linux box, that makes go test ./... blow up if pkg-config cant find libpostal, even though the HTTP fallback is already there.

This makes the native path opt-in via -tags libpostal. Default builds use the fallback now, tagged builds still use the cgo backend. Small fix, but it saves a dumb install footgun.

Repro

go env CGO_ENABLED
go test ./...

Before

Package 'libpostal', required by 'virtual:world', not found

Checks

go test ./...
go list -f '{{.GoFiles}} {{.CgoFiles}}' ./internal/libpostal
go list -tags libpostal -f '{{.GoFiles}} {{.CgoFiles}}' ./internal/libpostal

On this machine the default path now picks pure_go.go, and the tagged path still picks cgo_specific.go.