Maintainer decision on the FASTTIER review: drop the "Daybreak/cyber aliases
offer no speed tiers" rule this stack introduced. It flipped
model_supports_fast_mode to False for gpt-daybreak-* and gpt-5.6-cyber, which
also hid /fast and the Desktop Fast option for models that accepted Priority
before. Restore the pre-stack predicate verbatim, make the kept invariant
assert Daybreak is fast-but-not-ultrafast (the proxy route stays the no-speed
case) and remove the Daybreak sentences from the speed docs.
db68ebd223