fix: require writable external buffer storage

[GrapeBaBa]

Motivation

External ArrayBuffer and Buffer constructors accepted []const u8 and removed constness before exposing the backing memory to JavaScript. JavaScript can mutate that memory, so immutable storage such as string literals could be written through, causing undefined behavior.

Callers with immutable input also need an owning copy API instead of being pushed toward the zero-copy external constructors.

Description

• Require []u8 for external ArrayBuffer and Buffer backing storage and remove the internal @constCast operations.
• Reject immutable byte slices used with the .external_buffer return hint.
• Add createArrayBufferCopy for immutable input and document the existing Buffer copy behavior.
• Add integration coverage for independently writable copies, empty ArrayBuffers, and mutable external Buffer sharing.

Validated with pnpm build, pnpm test:zig, and the hello-world Vitest suite. The full JS suite retains the pre-existing macOS musl-target failure in examples/targets/mod.test.ts.