build: pin swift-subprocess dependency

[julianschiavo]

• Replace branch: "main" with from: "0.3.0" for the swift-subprocess dependency in Package.swift
• Tag 0.3 points to the same commit already resolved (ba5888a), so no functional change
• Ensures reproducible builds for downstream consumers

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Note

Low Risk
Low risk: this only changes SwiftPM dependency metadata from tracking main to a tagged 0.3.0 release, improving reproducibility without altering application code.

Overview
Switches the swift-subprocess SwiftPM dependency from branch: "main" to a versioned constraint (from: "0.3.0") in Package.swift.

Updates Package.resolved accordingly to record swift-subprocess as 0.3.0 (same pinned revision), making downstream builds more reproducible.

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