Brain: prefetch next task resources during current build

Brain evaluates the next goal only AFTER the current construction completes. If resources are short then, it orders a transport and idles until it lands. Between celestials in different systems that is 30+ minutes of a fully idle planet.

Measured on one celestial over a 7-hour window:

building time 5h14m
idle on transports 1h50m = 26%
waits observed 3x, each 33-35 min

Same cycle every time: task ends -> goal computed -> resource missing -> transport ordered -> ~34 min of nothing. The wait is just flight time, so it is paid again and again.

Suggested change: evaluate the next goal at the START of a construction and dispatch the transport immediately, so it lands before the current task ends. No extra fleet slots - same transports, sent earlier.

Scales with planet count and how spread out the account is, so it hits hardest on accounts that lean on Brain most.

Optional: config flag (PrefetchNextTask + lookahead depth) to keep current behaviour.