Progression: start with a hammermill only, unlock sorters one at a time #3

The player wants to start with a single hammermill and nothing else, then buy a sink-float tank for a first refining step, then one major sorter at a time (magnet, zig-zag air classifier, eddy current separator, screen, sensor sorter) until the full plant exists.

Work:

• Change STARTER_MACHINES in js/data.js to ['hammer'] and tune START_BANK so the first sorter is reachable in two or three batches.
• Re-price the separators so the intended order is the natural one: sink-float, magnet, air classifier, eddy current, screen, sensor sorter.
• Replace the current "Starter yard" preset with a hammermill-only line on end-of-life vehicles; the mixed shred must still sell for a small margin so the first batches earn.
• Add a "next purchase" hint on the bank panel: what the next sorter would add to margin on the current feed (the sim can compute this by trial-adding the machine).
• Update the help text and README.