New writer API, batch body adding bindings

[kcbanner]

• Add AddBodiesAbort / AddBodiesFinalize / AddBodiesPrepare
• Update the reader / writer to the new API

[Copilot]

Pull Request Overview

This PR updates the Zig physics library to support a new batch body adding API and modernizes I/O handling. The changes introduce a three-phase body addition system (Prepare/Finalize/Abort) for better performance when adding multiple bodies at once, and update the library to use newer Zig I/O APIs.

• Adds new batch body operations: AddBodiesAbort, AddBodiesFinalize, and AddBodiesPrepare
• Updates I/O handling from deprecated AnyWriter/AnyReader APIs to modern std.Io.Writer/std.Io.Reader
• Modernizes ArrayList initialization patterns and argument ordering
• Updates minimum Zig version requirement to 0.15.1

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/zphysics.zig	Main implementation changes including new batch API bindings, I/O modernization, and test updates
libs/JoltC/JoltPhysicsC.h	C header additions for new batch body interface functions
libs/JoltC/JoltPhysicsC.cpp	C++ implementation of the new batch body operations
build.zig.zon	Updates minimum Zig version requirement

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}