fix(diagnose): correct worked-example arithmetic (round-2 convergence)

Round-2 adversarial audit caught a real arithmetic contradiction in the worked
example: '3 duplicate rows per id' implies ~4x inflation (12,400 -> ~49,600),
but the example showed 12,400 -> 12,933. Reframed to 'duplicate covars rows for
a subset of ids -> +533 rows', which is internally consistent and matches the
displayed merge output. 'many-to-many merge' kept (correct for a panel: many
periods per id x duplicated covars per id). Round 2 is now dry.

Author: pedrohcgs

.claude/skills/diagnose/SKILL.md

@@ -127,8 +127,9 @@ A staggered-DiD `ATT` jumped from `−0.043` to `−0.071` after a data refresh;
 nrow(panel)                         # 12,400  (expected)
 nrow(merge(panel, covars, by="id")) # 12,933  <-- inflated! a many-to-many merge
 
-# Root cause: covars has 3 duplicate rows per id after the refresh; the join
-# fans 12,400 -> 12,933, re-weighting the ATT toward the duplicated units.
+# Root cause: the refresh left duplicate covars rows for a subset of ids; the
+# join fans those ids out, 12,400 -> 12,933 (+533 rows), re-weighting the ATT
+# toward the duplicated units.
 
 # Phase 5 — minimal fix at the root (dedup the key), NOT a downstream row filter:
 covars <- covars[!duplicated(covars$id), ]