HotSpot[ByteStream]: Split ByteStream into Reader and Writer

[ColinLeeo]

ByteStream is currently used for two fundamentally different purposes:
(1) a write-oriented, growable buffer optimized for encoding and frequent small writes, and
(2) a read-only wrapper over an existing buffer via wrap_from().
These two usages have very different requirements, but are implemented in a single class.

This mixed design complicates the implementation and blocks write-side optimizations such as fixed (power-of-two) page sizes and compile-time specialization, while also forcing the read path to carry unnecessary paging and write-related state. Splitting ByteStream into explicit Reader and Writer types clarifies responsibilities, simplifies invariants, and enables better performance and maintainability.