docs(short): el ejemplo que rompe una regla enseña a romperla

Segunda vez que aparece la misma lección sobre el ejemplo del prompt.

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

docs/shortsmith-phases-4-6.md

@@ -97,6 +97,24 @@ What actually landed, and where it differs from the plan below:
   redacted." costs three quarters of a second that a characters-only model gives away.
   Estimates now land within half a second of the three rendered MP4s. The lesson is the
   older one restated: a constant taken from one sample is a guess with a decimal point.
+- **The spec writer was under-declaring because the prompt asked for three things that
+  could not all be true** (2026-08-13). Every narrated spec the bot had written declared
+  less time than its own narration needed on three or four shots out of five or six —
+  3.8 to 8.2 seconds of drift per Short. The video still came out the right length,
+  because `plan()` grows the shot; what was wrong was that the spec described a visual
+  rhythm that never rendered. The cause was not a lazy model. The prompt asked at once
+  for lines of up to 18 words, shots of at most 6 seconds, and enough declared time for
+  the line — and 18 words need 7.3 s, so the set is unsatisfiable. The model broke the
+  only one of the three that nothing checked. Three fixes, all deterministic and none
+  costing a generation to find: the taught rule got the per-sentence term it was missing
+  (`words/2.75 + 0.5` fell short on 14 of the 28 measured lines, by up to 2.27 s — so
+  even perfect obedience under-declared); the word cap is now *derived* from the shot
+  cap rather than written by hand, so the contradiction cannot come back; and the worked
+  example, which violated its own rule on two of its six lines, was cut to obey it. That
+  last one is the lesson worth keeping: **the example is the strongest signal in the
+  prompt, so an example that breaks a rule teaches the breakage**, whatever the prose
+  says. It is the same finding as "el ejemplo del prompt habla, y por eso los specs
+  vuelven a hablar", arriving a second time.
 
 Original plan, kept for the record: