Say what the two measurement worktrees are for now

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

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -657,9 +657,10 @@ git worktree add -b split/19-name /home/bob/repos/iris-pr19 upstream/main
 
 `/home/bob/repos/iris-pr18` is the live one. Every other `iris-pr*` worktree
 holds a merged branch; they are readable references, not places to build.
-`/home/bob/repos/iris-perf2` is detached at `b1b3eca` and holds the text-cache
-prototype and `tests/fair.rs`; `iris-size-old` is `43ce8c7` with the old
-patched counters, and is how the pre-#16 column above was measured.
+`/home/bob/repos/iris-perf2` is detached at `b1b3eca`, built, and is the
+"before" side of any measurement of the last two commits; `iris-size-old` is
+`43ce8c7` with the old patched counters, and is how the pre-#16 column above
+was measured. Copy `tests/revision_cost.rs` into either to compare.
 
 ## Cautions