[Balance Idea]: Marginally Reduce cost and BT of T3 mobile Arty, increase cost of Spearhead so they both end up with 18 mass drain

### Intended Change

T3 mobile arty
800->720m
8000->7200E
4300->3600BT

Spearhead
400->480M
3000->3600E

This makes both of their mass drains 18m/s from a T3 factory with 90bp. Shifts the cost ratio of the two options from t3 arty is 2x the cost of spearhead to t3 arty is 1.5x the cost.

The reduced BT makes it less of a sacrifice to construct T3 arty (40 seconds not building direct combat vs 48 seconds)

### Reasoning

Its been clear for a while the Spearhead is undercoated and T3 Marty does not really have much a niche, with sniper bots, fat boys, spearheads and T2 stationary arty all stepping on its toes too much.

This is not an end all be all change, but just to get the ball rolling. Hopefully creating impetus for more players to build and test T3 Marty.

### Expected behavior

Cost/BT of all four T3 Marty's adjusted and Spearhead cost adjusted.

Locate the Lua unit definitions for the T3 mobile artillery and Spearhead, then review their current mass, energy, and build-time values. Compare the listed changes with the other T3 artillery units and confirm the adjusted values produce the stated 18 mass drain and expected balance behavior.

Written by the indexing model from the issue text.