refactor: address review nits for asset key/value refactoring

[Copilot]

• Create utility procedure load_asset_key_and_value in asset.masm
• Update epilogue.masm to use the utility procedure
• Update prologue.masm to use the utility procedure
• Update B2AGG.masm to use explicit ASSET_PTR constant
• Update faucets/mod.masm to use explicit ASSET_PTR constant
• Update swap.masm to use explicit ASSET_PTR constant
• Add test for merging two different assets in test_asset_vault.rs
• Keep variable name as asset in tx_event.rs (Asset type represents the whole asset, not just value)
• Fix stack manipulation in load_asset_key_and_value (movup.8 instead of movup.5)
• Fix stack index in prologue.masm (dup.1 instead of dup.4)
• Fix stack index in epilogue.masm (dup.1 instead of dup.4)
• Run cargo fmt to fix all formatting issues

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[mmagician]

@copilot run make lint and make sure CI passes

[Copilot]

@copilot run make lint and make sure CI passes

Done! Ran cargo fmt to fix formatting issues. The lint checks should now pass.

Note: The make lint command failed locally due to missing typos tool (requires Rust 1.91+, but we have 1.90). However, I've fixed the main formatting issues that were causing the CI lint workflow to fail.

Fixed in commit 7a57963.

[PhilippGackstatter]

Looks great, thanks! Left one small comment.

[PhilippGackstatter]

Nice improvements, thank you!