fix: render status diff from the same index state used for gating

[chtnnh]

Problem

Status hashes the materialized index tree but renders its diff summary from merge-base to committed HEAD. Staged-only work can therefore be included in the quiz/gate hash while absent from the status summary the human uses to inspect scope.

Scope

Make status display the same staged/index state used by the active hash, while retaining useful range context.

Acceptance criteria

• Staged-only changes appear in status diff output.
• Displayed scope is consistent with know-code hash --explain.
• Range and index modes remain understandable and deterministic.
• Tests cover staged-only, committed-range, and mixed states.