feat: implement nested structs and zero-allocation fixes

[suckserberg]

Summary

This PR introduces core API enhancements and critical performance fixes to the serialization pipeline. The most significant additions are the Satset.struct() function for nested schema support and human-readable type aliases to improve developer experience.

Technical Implementation

• [init.luau]: Implemented Satset.struct() which wraps raw schema tables into reusable Type objects. This allows for deep nesting and better composability.
• [SchemaCompiler]: Added zero-field support and a safe fallback for 4+ fields in buildFixedDecoder using table.unpack, ensuring robust handling of complex schemas.
• [Types]: Added long-form aliases (float64, uint8, etc.) as the recommended naming standard for better readability.

Changelog Entry

• Core: Implementation of nested structs and human-readable type aliases. Enables complex data structures with improved schema clarity.

Type of Change

• Feature
• Bug Fix
• Performance Optimization
• Documentation Update
• Maintenance / Chore

Verification and Standards

• I have adhered to the Development Patterns.
• I have verified that all CI/CD linting and formatting checks pass.
• I have confirmed the changes within a Rojo-supported environment.
• I have updated the technical documentation for any API or architectural changes.

[protheeuz]

LGTM. 👍🏻