Gas estimation for blob submission

[tac0turtle]

Summary

Implement deterministic gas estimation for MsgPayForBlobs transactions.

Parent: #4

Design

Recreate the gas estimation formula from scratch rather than importing celestia-app:

gas = gasToConsume(blobSizes, gasPerBlobByte) + (txSizeCostPerByte * bytesPerBlobInfo * numBlobs) + pfbGasFixedCost

Constants (from celestia-app, may need updating per network version):

• pfbGasFixedCost = 75,000
• bytesPerBlobInfo = 70
• gasPerBlobByte = 8
• txSizeCostPerByte — governance parameter, query from chain or configure

Gas price

• Configurable default gas price in config
• Optional multiplier/buffer (e.g., 1.1x) to reduce "insufficient fee" rejections
• No dependency on celestia-node's gas estimator gRPC service
• Optionally query QueryMinimumGasPrice() from a celestia-app RPC endpoint as a floor

[submission]
gas_price = 0.002         # utia per gas unit
gas_price_buffer = 1.1    # 10% buffer over minimum
max_gas_price = 0.2       # safety cap

Requirements

• Pure function: given blob sizes, return gas estimate
• No network calls required (deterministic)
• Unit tests validating against known celestia-app outputs
• Configurable constants for network upgrades

References

• celestia-app x/blob/types/payforblob.go — DefaultEstimateGas(), GasToConsume()