docs: add documentation for JS-compatible number types

[Lil-Duckling-22]

Motivation

The public JS-compatible number types (JsU64, JsU128, JsI64, JsI128, JsF32, JsF64) lacked documentation, making it unclear when and how to use them for fuzzing with JavaScript implementations.

Solution

Added module-level and type-level documentation explaining:

• Purpose: JS-compatible serialization for cross-language fuzzing
• Behavior: large integers serialize as strings, floats as hex-encoded bytes
• Usage context: required for fuzzing targets that interface with JS implementations

[github-actions]

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