Enable missing_docs rule and add missing API documentation

[ctreffs]

Context & Summary

This PR enables the missing_docs SwiftLint rule in .swiftlint.yml to enforce documentation for public APIs and adds missing documentation comments across FirebladeMath.

What Changed

• .swiftlint.yml: Enabled the missing_docs rule under opt_in_rules.
• Sources/FirebladeMath/Quat/Quaternion+Identity.swift: Added SwiftDoc comments for Quat4d.identity and Quat4f.identity.
• Sources/FirebladeMath/Quat/Quat4f+Euler.swift: Added SwiftDoc comments for Euler conversion functions (fromEulerAngles_*, quaternionToEulerAngles_*), initializer init(pitch:yaw:roll:), and eulerAngles, pitch, yaw, roll accessors.
• Sources/FirebladeMath/Functions/radians.swift: Added SwiftDoc comments for radians(_ degrees: Vec3f).

Reviewer Guidance

Testing Steps

1. Run make lint to verify SwiftLint passes without warnings.
2. Run make test to verify all test suites compile and pass.

Risk of Regression

• Level: Low
• Rationale: Changes are strictly additions of SwiftDoc comments and enabling a SwiftLint rule. No operational logic was modified.

Focus Areas

• Public API documentation coverage in Quat4f+Euler.swift.