Implement Array.flat, flatMap, and splice

[memburg]

Goal

Add the next high-impact Array methods after callback basics.

Why

These methods are common in real-world scripts and complete many practical array workflows.

Scope

Implement support for:

• Array.prototype.flat
• Array.prototype.flatMap
• Array.prototype.splice

Plan

• Add runtime implementations for each method.
• Ensure expected mutation/non-mutation behavior.
• Add specs for typical usage and argument validation.
• Update type/reference docs.

Out of Scope

• Full edge-case parity for sparse arrays in first pass.