ewd998: raise EWD998_proof budget from 4 to 6 minutes

The 4-minute budget I set in #218 is marginal on the macOS runners. It
passes most of the time and was killed at the wall in run 31000876116,
with the proof itself unaffected:

  54195 Killed: 9  timeout --signal=KILL 4m ... EWD998_proof.tla

Measured locally with tlapm 4600b24 and --stretch 5: 86 seconds, all 850
obligations proved. CONTRIBUTING suggests budgeting up to twice the local
time, which is where 4 came from; the macOS runner needed more than 2.8x.
Six minutes gives roughly 4x local headroom.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Vasilis Nasopoulos <vasilis_nasopoulos@hotmail.com>

Author: 2 people

specifications/ewd998/manifest.json

@@ -162,7 +162,7 @@
       "features": [],
       "models": [],
       "proof": {
-        "maxRuntimeMinutes": 4
+        "maxRuntimeMinutes": 6
       }
     },
     {