Refactor Missile direction/frame group handling

[kphoenix137]

PR Description: Clarify and Encapsulate Missile Frame Group Handling

Problem

The Missile struct contains a member _mimfnum, originally defined as an int, which is used inconsistently across the codebase in three distinct ways:

• As a raw int (e.g., for animation frame group selection)
• As Direction (8-directional enum)
• As Direction16 (16-directional enum, used for finer visual rotation)

This loose usage made the intent behind _mimfnum ambiguous and error-prone, requiring frequent manual casting and encouraging magic numbers.

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Solution

This PR improves encapsulation and usability of _mimfnum by:

• Making _mimfnum a private member of Missile
• Introducing explicit getters and setters to clarify usage:
  • setDirection(Direction)
  • setDirection(Direction16)
  • getDirection(), getDirection16()
  • setFrameGroupRaw(int)
  • getFrameGroupRaw()
  • setFrameGroup<Enum>() and getFrameGroup<Enum>()
  • setDefaultFrameGroup() (shorthand for setFrameGroupRaw(0))

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Benefits

• Improved readability – Call sites now express intent clearly, e.g., missile.setDirection(Direction::SouthEast)
• Type safety – Template-based frame group accessors eliminate the need for static_casts at call sites
• Encapsulation – Prevents accidental misuse or incorrect assignments to _mimfnum
• More intuitive code – Developers working with missiles can reason about visuals/direction/frame groups more easily

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Example Usage

Before:

missile._mimfnum = static_cast<int>(Direction::South);

After:

missile.setDirection(Direction::South);

Or for custom frame groups:

missile.setFrameGroup<GuardianFrame>(GuardianFrame::Attack);

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This refactor lays a strong foundation for extending missile behavior safely and clearly while preserving compatibility with save/load systems that still require reading and writing _mimfnum as an int.