Refactor: make minUsagePadding a constant

[htoyoda18]

Summary

The variable minUsagePadding is currently defined as a package-level variable, but it represents a fixed configuration value that never changes at runtime.
It would be more idiomatic and safer in Go to define it as a constant.

Current Code

var minUsagePadding = 25

Proposed Change

Change it to:

const minUsagePadding = 25

Rationale

• The value is immutable and conceptually a compile-time constant.
• Using const makes the intention clearer to readers and contributors.
• It prevents any accidental modification at runtime and improves code safety.

Impact

This change is fully backward compatible and has no functional impact on behavior.
It only improves clarity and maintainability of the codebase.