P1: fix Zig FFI to build and match the Idris2 ABI (#38)

`zig test src/main.zig -lc` failed to compile under Zig 0.14.0 with 9
"unused function parameter" errors. In Zig 0.14 an unused function
parameter is a hard error; the leading-underscore naming convention does
NOT exempt a parameter from the unused-parameter check. The affected
stub functions were `futharkiser_compile`, `futharkiser_compile_file`,
`futharkiser_execute`, and `futharkiser_transfer`.

Fix (house idiom, per alloyiser exemplar): rename the underscore-prefixed
parameters to plain names and discard them with `_ = name;` statements at
the top of each function body. This keeps the not-yet-implemented stubs
valid while preserving the exact C-ABI signatures.

ABI conformance (Idris2 is the source of truth) is unchanged and verified:
  - All 14 `C:futharkiser_*` symbols in
    src/interface/abi/Futharkiser/ABI/Foreign.idr have a matching
    `export fn` (no missing, no extra).
  - The Result enum integer values (ok=0, error=1, invalid_param=2,
    out_of_memory=3, null_pointer=4, compilation_failed=5,
    backend_unavailable=6, shape_mismatch=7) match
    Types.idr `resultToInt` exactly.
  - Exported symbol names and arities are unchanged.

Verification:
  - `zig test src/main.zig -lc`  -> all 9 tests pass, 0 errors/warnings.
  - `idris2 --build futharkiser-abi.ipkg` -> exit 0.

Only src/interface/ffi/src/main.zig was changed.


Claude-Session: https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

src/interface/ffi/src/main.zig

@@ -154,9 +154,11 @@ export fn futharkiser_free(raw_handle: ?*anyopaque) void {
 /// Returns a kernel handle on success, or null on compilation failure.
 export fn futharkiser_compile(
     raw_handle: ?*anyopaque,
-    _source: [*:0]const u8,
-    _backend_id: u32,
+    source: [*:0]const u8,
+    backend_id: u32,
 ) ?*anyopaque {
+    _ = source;
+    _ = backend_id;
     const handle: *FutharkiserHandle = @ptrCast(@alignCast(raw_handle orelse {
         setError("Null library handle");
         return null;
@@ -180,9 +182,11 @@ export fn futharkiser_compile(
 /// Compile a Futhark source file to a GPU kernel.
 export fn futharkiser_compile_file(
     raw_handle: ?*anyopaque,
-    _file_path: [*:0]const u8,
-    _backend_id: u32,
+    file_path: [*:0]const u8,
+    backend_id: u32,
 ) ?*anyopaque {
+    _ = file_path;
+    _ = backend_id;
     const handle: *FutharkiserHandle = @ptrCast(@alignCast(raw_handle orelse {
         setError("Null library handle");
         return null;
@@ -205,9 +209,11 @@ export fn futharkiser_compile_file(
 /// Execute a compiled GPU kernel by entry-point name.
 export fn futharkiser_execute(
     raw_handle: ?*anyopaque,
-    _kernel_handle: ?*anyopaque,
-    _entry_point: [*:0]const u8,
+    kernel_handle: ?*anyopaque,
+    entry_point: [*:0]const u8,
 ) Result {
+    _ = kernel_handle;
+    _ = entry_point;
     const handle: *FutharkiserHandle = @ptrCast(@alignCast(raw_handle orelse {
         setError("Null library handle");
         return .null_pointer;
@@ -302,10 +308,13 @@ export fn futharkiser_free_buffer(
 /// Transfer data between memory spaces (host <-> device).
 export fn futharkiser_transfer(
     raw_handle: ?*anyopaque,
-    _src_handle: ?*anyopaque,
-    _dst_handle: ?*anyopaque,
-    _flags: u64,
+    src_handle: ?*anyopaque,
+    dst_handle: ?*anyopaque,
+    flags: u64,
 ) Result {
+    _ = src_handle;
+    _ = dst_handle;
+    _ = flags;
     const handle: *FutharkiserHandle = @ptrCast(@alignCast(raw_handle orelse {
         setError("Null library handle");
         return .null_pointer;