docs: explore sufficiency -- the metric, its rules, what it caught (CG-8)

Records what each bucket means and which fix it points at, the four rules that
keep the classification honest (same-message calls, bookkeeping tools, subagent
threads, earlier-explore files), and the three real transcripts it was
hand-checked against -- including the excalidraw canvasNonce run, where it
independently found the data-flow frontier CLAUDE.md already documents: 0%
sufficient, without being told what to look for.

Also states what it does NOT say: sufficient is not correct, one Read is a vote
rather than a proof, and bucket 1 is ambiguous by construction.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/benchmarks/residual-context-occupancy.md

@@ -194,6 +194,7 @@ is a baseline across the 7 README repos to compare future changes against.
   only its summary returns to the parent. Runs that delegate are measured on the
   parent's window alone.
 - **Occupancy is not sufficiency.** A small residual is only good if the answer
-  was still right. This metric says nothing about answer quality — that is
-  CG-8's job (sufficiency) and CG-9's (how much of the returned bytes the answer
-  actually used).
+  was still right. This metric says nothing about whether the response was
+  *enough* — that is [explore sufficiency](explore-sufficiency.md), which every
+  run now prints alongside this block — nor about how much of the returned bytes
+  the answer actually used (CG-9).