fix(diagnostics): classify external import edges

[cgoulart21]

Summary

• classify intentional out-of-corpus imports_from -> ref_* edges separately
  from malformed internal endpoints
• expose the new external_import_edges diagnostic count and explain it in the
  text report
• add an end-to-end JavaScript regression that also keeps a genuinely dangling
  internal edge visible

Closes #2130.

Root cause

The JavaScript/TypeScript resolver deliberately namespaces unresolved external
packages as ref_* so they cannot alias onto unrelated in-corpus nodes. The
builder intentionally drops those out-of-corpus edges, but
diagnose_extraction() categorized every absent endpoint as dangling. This made
expected dependency evidence indistinguishable from corrupt internal edges.

Validation

• pytest tests/test_multigraph_diagnostics.py tests/test_phantom_external_import.py tests/test_ts_import_require.py -q
  • 40 passed
• ruff check graphify/diagnostics.py tests/test_multigraph_diagnostics.py
  • passed
• pyright --pythonpath .venv/Scripts/python.exe graphify/diagnostics.py tests/test_multigraph_diagnostics.py
  • 0 errors, 0 warnings
• public ClawSCAD reproduction
  • 29 external import edges
  • 0 dangling endpoint edges for the JavaScript/TypeScript corpus
• full Windows suite
  • 3,501 passed, 32 skipped
  • 59 unrelated environment/platform failures (symlink privilege, POSIX path
    assumptions, sandboxed global install paths, and optional OpenAI dependency)