log the fifth measurement defect: a count that was not counting

git diff errored to stderr on a malformed revision while wc -l counted empty
stdout, producing three confident IDENTICAL results that meant nothing. Had the
promoted trees actually differed, I would have reported the promotion clean.

The correct check is not 'how many files differ' but 'is the tree object the
same object' -- all three share hash 2acd9374.

All five defects are now visibly one shape: reading a PROXY instead of the
thing. One file instead of the operation, a variable name instead of the shape,
a scope instead of the whole, a pipe's exit instead of the program's, a line
count instead of object identity.

docs/v1/experiments/findings/measurement-defects.md

@@ -1,7 +1,9 @@
 # Defects in my own measurement
 
-Recorded because the pattern is the point: **four of these, all the same shape —
-searching by name or scope instead of by the operation itself.** Each was caught
+Recorded because the pattern is the point: **five of these, and every one is the same shape —
+reading a proxy instead of the thing.** A file instead of the operation, a
+variable name instead of the shape, a scope instead of the whole, a pipe's exit
+instead of the program's, a line count instead of the object identity. Each was caught
 by running something, never by reading.
 
 ### 1. Scoped the search to one file
@@ -32,6 +34,25 @@ timeout 10 /tmp/leakrun 2>&1 | head -2; echo "exit=$?"     # reports head's exit
 Reported `exit=0` for a program that was returning **139 (SIGSEGV)**. I nearly
 recorded a segfault as a clean run.
 
+### 5. Read a count that was not counting
+
+Comparing the three promoted branches:
+
+```bash
+for pair in "dev stage" ...; do set -- $pair
+  n=$(git diff --stat origin/$1 origin/$2 | wc -l)     # git errored to STDERR
+  ...                                                   # wc counted empty STDOUT
+```
+
+`git diff` failed on a malformed revision, wrote its error to stderr, and `wc -l`
+counted zero lines of stdout. Three confident `IDENTICAL` results, all
+meaningless. **Had the trees actually differed, I would have reported the
+promotion clean.**
+
+Redone correctly, the three trees share one hash — `2acd9374` — which is the
+check that should have been run first: not "how many files differ" but "is the
+tree object the same object".
+
 ### And one that was not a measurement defect but a method defect
 
 One cycle was run **without committing predictions first** — see