Add Base64String (primitives)

[vhrcgcom]

Description

Implement Base64String as a ValueObject in the primitives module.

Spec: valid base64-encoded string

Implementation checklist

• Create src/primitives/base64string.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/primitives/mod.rs and prelude
• Unit tests: valid input · empty input · invalid format · normalisation
• Doc comment with # Example block
• Update status in ROADMAP.md from ⬜ to ✅

Implementation detail

Input / Output

	Type	Alias
Input	String	Base64StringInput
Output	String — the validated base64 string as-is	Base64StringOutput

Normalization

Trim surrounding whitespace.

Validation

• Must be valid standard Base64 (alphabet A–Z, a–z, 0–9, +, /, with = padding).
• Validate using the base64 crate (add as optional dependency to the primitives feature) — attempt to decode; if it succeeds the input is valid.
• Padding must be correct: length must be a multiple of 4 (after trimming).

Extra methods

• decode() -> Vec<u8> — returns the decoded raw bytes. This operation always succeeds since construction already validates the encoding.

References

• ROADMAP.md
• CONTRIBUTING.md