Add explicit ASCII-based int values to Alpha3Code enum

[Copilot]

Alpha2Code encodes its int values by concatenating ASCII codes of the two characters (e.g., AF → 6570). Alpha3Code had no explicit values, defaulting to sequential ints. This applies the same scheme to Alpha3Code.

• Alpha3Code.cs: Changed enum to : int, assigned each member its ASCII-concatenated value (e.g., AFG = 657071, USA = 858365)
• Alpha3Code.cs: Added <remarks> XML doc explaining the value scheme, matching Alpha2Code
• CountryTest.cs: Added test validating all 250 members have correct ASCII-derived values

// Before
public enum Alpha3Code
{
    AFG,
    ALA,
    ...
}

// After
public enum Alpha3Code : int
{
    AFG = 657071,  // A(65) + F(70) + G(71)
    ALA = 657665,  // A(65) + L(76) + A(65)
    ...
}

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