fix(railgun): declare libc open as variadic (rustc 1.99 CI build break)

[fivetide]

Summary

rustc 1.99, which CI installs through dtolnay/rust-toolchain@stable, now denies invalid_runtime_symbol_definitions: an extern declaration of a symbol the standard library itself links must match that symbol's signature. railgun's npu module declared open with a non-variadic signature, so every workspace build on beta (the build, test and clippy jobs) fails at railgun:

-        pub fn open(path: *const u8, flags: i32) -> i32;
+        pub fn open(path: *const core::ffi::c_char, flags: i32, ...) -> i32;
 ...
-        let fd = unsafe { sys::open(b"/dev/accel/accel0\0".as_ptr(), sys::O_RDWR | sys::O_CLOEXEC) };
+        let fd = unsafe { sys::open(c"/dev/accel/accel0".as_ptr(), sys::O_RDWR | sys::O_CLOEXEC) };

Evidence

• Before (cargo +1.99.0 build --release --locked -p railgun --features npu, on beta):

  error: invalid definition of the runtime `open` symbol used by the standard library
    --> crates/railgun/src/npu/mod.rs:24:9
     = note: expected `unsafe extern "C" fn(*const i8, i32, ...) -> i32` (for the current target)
             found    `unsafe extern "C" fn(*const u8, i32) -> i32`
  

  This is the same error as the CI build job on beta 3653dd6d3 (run 37479462665).
• After: cargo +1.99.0 and cargo +1.97.1 build --release --locked -p railgun --features npu both finish.
• With this fix applied, the build and clippy jobs pass under 1.99.0 locally, mirroring ci.yml.

The test job still has red that doesn't come from this change: six tests in peacemaker-ir and hipfire-isa run a hardcoded /opt/rocm/core-10.0/lib/llvm/bin/llvm-mc, which a GitHub runner doesn't have. The ungated_examples ratchet also fails on beta. Neither is touched here.

Merge Danger

Door: two-way. The change touches one declaration and its single call site. The ABI is unchanged: the C open is variadic, and the old declaration simply omitted the variadic part.

Blast Radius: railgun with the npu feature only.