upgrade-all: at the tail, fill slots with two heavies rather than serialize

Per operator: always fill all CAP slots. Heavy/light alternation only spreads
heavies across waves while a light is available; once only heavies remain, run
two-per-wave (capacity is the tuned ceiling) instead of one-per-wave.

.claude/skills/upgrade-all/SKILL.md

@@ -164,8 +164,10 @@ peak memory stays bounded while both slots stay busy. Classify from the survey's
 - **LIGHT / moderate** (one small image or a few light services): custom-html*, cryptpad, n8n,
   uptime-kuma, lasuite-docs, lasuite-meet, keycloak, mailu, plausible.
 Build the wave order by drawing one HEAVY + one LIGHT per wave (capacity 2) until a bucket empties,
-then pair the remainder. For `CAP>2`, cap heavies per wave at ~`CAP/2`. If only heavies remain, run
-them one-per-wave (effectively sequential) rather than risk an OOM.
+then pair up the remainder. **Always fill all `CAP` slots** — once only heavies are left, run two (or
+`CAP`) heavies per wave rather than leaving a slot idle; the box is tuned for `CAP` concurrent builds.
+The heavy/light alternation is only to *spread* heavies across waves while a light is available, not a
+hard cap. For `CAP>2`, spread heavies across waves where possible but still fill the slots.
 
 Note (wave barrier): a wave waits for its slowest recipe before the next starts, so the light slot
 idles while its heavy wave-mate finishes — accepted for the weekly run. Do NOT maintain a rolling