Update EVM bridge FLIP to clarify storage-based fee model

[joshuahannan]

Summary

• Adds a fee model callout at the top of the Case Studies section making the principle explicit: bridge fees are only charged when an operation causes the bridge account to store an asset long-term
• Cadence-Native Cadence → EVM: replaces "flat fee + storage usage" with "storage consumed by escrowing the NFT in the bridge account"
• Cadence-Native EVM → Cadence: adds a note explaining no fee is charged (releasing from escrow doesn't add storage), removes the fee withdrawal step
• EVM-Native EVM → Cadence: clarifies fee is based on storage consumed by holding the ERC721 in the bridge's COA

Reflects the clarifications from onflow/flow-evm-bridge#202.

Test plan

• Review that the fee model callout accurately describes the implemented behavior in FlowEVMBridge.cdc
• Verify the per-path fee descriptions match the handler logic in the bridge contract

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