Simplified 2048, but reliably reaches 65k

[kywch]

Inspired by @drubinstein 's recent 2048 simplification, I did -- so 300 less lines of code in 2048.h for better perf.

• Used @drubinstein 's grid only obs, and value/pos embedding encoder
• Removed heuristic/snake rewards completely, and endgame training
• Simplified scaffolding curriculum
• Prepared for 131k tiles
• And, fixed wrong done signals.

Wandb curves:

• https://wandb.ai/kywch/pufferlib/runs/1v5kls7l?nw=nwuserkywch
• https://wandb.ai/kywch/pufferlib/runs/v0yym0pu?nw=nwuserkywch

Evaluation (this is one of the best runs. Perf changes from seed to seed, but hits 65k reliably):

    Num episodes: 95611
    Max tile avg: 40980.9
    Episode length -- Avg: 26792.1, Max: 37442.2
    Merge score -- Avg: 779238.6, Max: 997571.8
    Reached 32768 prob: 84.88 %
    Reached 65536 prob: 33.96 %
    Reached 131072 prob: 0.00 %   

Git commits are weird, but changed files are included here.