P1: fix Zig FFI to compile under Zig 0.14.0

The Zig FFI layer failed to compile under Zig 0.14.0 with four
"pointless discard of function parameter" errors in
halideiser_execute_pipeline (src/interface/ffi/src/main.zig).

The parameters input_ptr, output_ptr, input_len, and output_len are
already read by the null-pointer and zero-length validation checks
earlier in the function body. Zig 0.14 rejects `_ = param;` discards of
parameters that are genuinely used elsewhere, so the four trailing
discards were redundant and erroneous. Removed them (replaced with an
explanatory comment); the unrelated discards in halideiser_autotune
stay, as max_trials/timeout_ms are genuinely unused there.

No other errors surfaced once the discards were removed.

ABI preserved: every C:<name> in Foreign.idr still maps to an
`export fn`, and the Result enum integer values (ok=0 .. dimension_
mismatch=7) are unchanged and continue to match resultToInt in Types.idr.

Verification:
  - zig test src/interface/ffi/src/main.zig -lc  -> all 9 tests pass
  - idris2 --build halideiser-abi.ipkg           -> exit 0

Only files under src/interface/ffi/ were changed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

Author: claude

src/interface/ffi/src/main.zig

@@ -424,10 +424,8 @@ export fn halideiser_execute_pipeline(
     //   1. Wrap input_ptr/output_ptr as halide_buffer_t
     //   2. Call the compiled pipeline function
     //   3. Return the result code
-    _ = input_ptr;
-    _ = output_ptr;
-    _ = input_len;
-    _ = output_len;
+    // NB: input_ptr/output_ptr/input_len/output_len are already validated above,
+    // so they must not be discarded (Zig 0.14 rejects pointless discards).
 
     clearError();
     return .ok;