feat(helm): extract Helm/Go-template named templates and include calls

Helm chart helpers (.tpl) and Go templates produced no semantic graph — just a
flat key flood. Wire the existing gotemplate grammar for real structure:

- .tpl files map to the gotemplate language (Helm _helpers.tpl).
- {{ define "chart.fullname" }} becomes a Function node named after the
  template (the name is a string literal, not an identifier, so it's resolved
  in a dedicated walker branch).
- {{ include "x" . }} and {{ template "x" . }} resolve to the referenced
  template name instead of the bare `include` builtin, so they form CALLS
  edges to the define'd Function. template_action is added to the call set.

Lets you trace which charts include which named templates. Tests cover define
-> Function and include/template -> CALLS, plus the .tpl mapping.

Refs #338