RFC: Clarify `mixed` contributor semantics (non-normative)

[jonathansantilli]

mixed is useful but ambiguous in edge cases (format-only edits, small touch-ups, major rewrites).

Problem

The spec does not clarify whether a universal threshold is expected, which can lead to inconsistent classification across producers.

Proposal

Add non-normative clarification:

1. Do not require a universal threshold for mixed.
2. Producers should apply internally consistent heuristics.
3. Producers may expose heuristic details via metadata for transparency.

Why This Should Be Added

1. Reduces interpretation drift while keeping flexibility.
2. Supports cross-implementation comparisons with clearer expectations.
3. Maintains a minimal core spec.

Compatibility

Clarification-only; no schema change required.

Scope

This is contributor-type interpretation guidance. It does not prescribe a single classification algorithm.