wait_until_connected fail-fasted on stall even when the mesh was all but
converged, abandoning the run with budget still unspent because one hard
pair straggled to come up. On the rekey-outbound-only topology this turned a
rare deep-node timing straggle (stacked discovery backoff + late bloom
propagation, which clears well inside the budget) into a false baseline RED.
Add a near_converged_slack threshold (default 2): when the number of
still-failing pairs is at or below the slack and the stall window elapses,
keep polling toward max_secs instead of bailing. A mesh genuinely far from
convergence still fast-bails on stall, and a never-converging pair still
hits the hard cap, so a real regression is never masked.
Add a self-contained behavioral test (wait-converge-test.sh) covering the
four contract cases: near-converged hold (with a slack=0 contrast that
proves the slack is what saves the run), far-from-converged fast-bail,
never-converging hard cap, and four-argument backward compatibility.
