Add ApiKey (net)

[vhrcgcom]

Description

Implement ApiKey as a ValueObject in the net module.

Spec: non-empty; masked Display shows only last 4 chars

Implementation checklist

• Create src/net/apikey.rs
• Implement ValueObject trait
• Add #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
• Export from src/net/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	ApiKeyInput
Output	String — the full key, stored internally	ApiKeyOutput

Normalization

Trim.

Validation

• Must not be empty after trimming.

Display

Display and masked() show only the last 4 characters, with the rest replaced by *. For keys shorter than 4 characters, mask all but the last character.

value() returns the full key — callers must treat this as sensitive data.

Extra methods

• last_four() -> &str — the last 4 characters of the key.
• masked() -> String — the masked representation, e.g. "********************abcd".

References

• ROADMAP.md
• CONTRIBUTING.md