FORWARD: clarify or implement reference-field path in _lead_dots (currently NotImplementedError) #13755

Describe the new feature or enhancement

While reviewing forward internals, I found an unimplemented branch in mne/forward/_lead_dots.py:

if rref is not None:
    raise NotImplementedError  # we don't ever use this, isn't tested

I propose clarifying this behavior by either implementing this reference-field path with tests, or explicitly documenting/raising a clearer unsupported-mode error.

Describe your proposed implementation

Preferred approach:

• confirm expected behavior with maintainers (support vs explicitly unsupported)
• if supported: implement the rref branch and add targeted tests
• if unsupported: replace the generic NotImplementedError with an actionable error message and add a short docs/changelog note

Describe possible alternatives

1. Leave as-is

• simplest, but keeps an ambiguous untested failure path

2. Implement immediately without alignment

• faster, but may conflict with intended forward-model design

Chosen approach (align first, then implement or explicitly mark unsupported) reduces risk.

Additional context

I checked related forward/report activity (#2828, PR #13722), which appears focused on report-level sensitivity-map integration, not this low-level _lead_dots branch. So this is intended as separate forward-core tracking.