Create unified Instruction protocol and fix inheritance

[TravisBumgarner]

Description

Several instruction classes don't properly inherit from a base class, breaking type inference:

• InstructionFeedRate has no parent class
• Instruction3DPrinterPlottingHeight has no parent class
• Instruction3DPrinterNavigationHeight has no parent class

Create a Protocol that all instruction classes implement, ensuring consistent interface.

Proposed Changes

Add to instruction module:

from typing import Protocol

class Instruction(Protocol):
    def to_g_code(self) -> str: ...
    def __str__(self) -> str: ...

Ensure all instruction classes either:

1. Inherit from a common base class, OR
2. Implement the Protocol interface

Acceptance Criteria

• Create Instruction protocol in instruction module
• InstructionFeedRate implements the protocol (add _BaseInstruction inheritance or ensure protocol compliance)
• Instruction3DPrinterPlottingHeight implements the protocol
• Instruction3DPrinterNavigationHeight implements the protocol
• Update TInstructionUnion type in _Layer.py to use the protocol
• All existing tests pass
• mypy passes with no new errors