Decide whether the settle loop can stop on this poll, and why.
Returns the SettleReason to exit with, or null to keep polling.
Priority: the adaptive floor gates everything; a declared+present beacon wins
over heuristics; otherwise the page must be non-busy AND hash-stable, and the
reason distinguishes "markers cleared" (we saw it busy, then it settled) from
a plain "stable"/"networkidle" quiescence.
Decide whether the settle loop can stop on this poll, and why. Returns the SettleReason to exit with, or
nullto keep polling.Priority: the adaptive floor gates everything; a declared+present beacon wins over heuristics; otherwise the page must be non-busy AND hash-stable, and the reason distinguishes "markers cleared" (we saw it busy, then it settled) from a plain "stable"/"networkidle" quiescence.