RamBlock::new uses libc::mmap and fails to compile on Windows

[luojia65]

RamBlock::new in memory/src/ram.rs currently uses libc::mmap directly:

pub fn new(size: u64) -> Self {
    // SAFETY: mmap with MAP_ANONYMOUS returns a fresh
    // zero-filled mapping or MAP_FAILED.
    let ptr = unsafe {
        libc::mmap(
            ptr::null_mut(),
            size as usize,
            libc::PROT_READ | libc::PROT_WRITE,
            libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
            -1,
            0,
        ) as *mut u8
    };
    assert!(
        !ptr.is_null() && ptr != libc::MAP_FAILED as *mut u8,
        "mmap failed for size {size}"
    );
    Self { ptr, size }
}

This implementation is Unix / Linux-specific and fails to compile on Windows.

Problem

The code assumes the availability of libc::mmap, libc::munmap, and related constants such as PROT_READ, PROT_WRITE, MAP_PRIVATE, MAP_ANONYMOUS, and MAP_FAILED. These are not available in libc on Windows, so the project cannot be built there.

Reproduction

On Windows PowerShell:

PS D:\RustSBI\machina> cargo build
   Compiling machina-core v0.1.0 (D:\RustSBI\machina\core)
   Compiling libc v0.2.183
   Compiling machina-memory v0.1.0 (D:\RustSBI\machina\memory)
error[E0425]: cannot find function `mmap` in crate `libc`
  --> memory\src\ram.rs:17:19
   |
17 |             libc::mmap(
   |                   ^^^^ not found in `libc`

error[E0425]: cannot find value `PROT_READ` in crate `libc`
  --> memory\src\ram.rs:20:23
   |
20 |                 libc::PROT_READ | libc::PROT_WRITE,
   |                       ^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `PROT_WRITE` in crate `libc`
  --> memory\src\ram.rs:20:41
   |
20 |                 libc::PROT_READ | libc::PROT_WRITE,
   |                                         ^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `MAP_PRIVATE` in crate `libc`
  --> memory\src\ram.rs:21:23
   |
21 |                 libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
   |                       ^^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `MAP_ANONYMOUS` in crate `libc`
  --> memory\src\ram.rs:21:43
   |
21 |                 libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
   |                                           ^^^^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find value `MAP_FAILED` in crate `libc`
  --> memory\src\ram.rs:27:44
   |
27 |             !ptr.is_null() && ptr != libc::MAP_FAILED as *mut u8,
   |                                            ^^^^^^^^^^ not found in `libc`

error[E0425]: cannot find function `munmap` in crate `libc`
  --> memory\src\ram.rs:52:19
   |
52 |             libc::munmap(self.ptr as *mut libc::c_void, self.size as usize);
   |                   ^^^^^^ not found in `libc`

For more information about this error, try `rustc --explain E0425`.
error: could not compile `machina-memory` (lib) due to 7 previous errors

Expected behavior

The project should either:

1. support Windows with a platform-specific implementation of RamBlock::new and Drop, or
2. explicitly restrict this code path to Unix platforms using cfg(unix) and provide a clearer unsupported-platform error.

Suggested direction

Introduce a small cross-platform RamBlock abstraction: use mmap on Unix, VirtualAlloc on Windows, a boxed-slice backend for Wasm, and a generic heap-based fallback for other targets. We can name them UnixRamBlock, etc.

Why this matters

At the moment, machina-memory does not compile on Windows because the RAM backend is hardcoded to a Unix-specific API.