P1: fix Zig FFI to build and match the Idris2 ABI

The Zig FFI in src/interface/ffi/src/main.zig failed `zig test src/main.zig
-lc` under Zig 0.14.0 and one test asserted behaviour the ABI contract does
not provide.

Errors found:

1. Compile error: `array_list.ArrayListAligned` no longer exposes
   `popOrNull`. `oblibeniser_undo_pop` called `h.undo_stack.popOrNull()`,
   which does not exist in Zig 0.14.0.

2. Test failure: the "compute and verify inverse" test finalised the
   operation with a null (0) post-snapshot pointer, so `post_snapshot`
   stayed null and `oblibeniser_compute_inverse` correctly returned
   `not_reversible` (per its contract: an inverse needs both pre- and
   post-state snapshots). The test asserted `ok`, so it failed.

Fixes:

1. Replaced `popOrNull()` with `pop()`, which in Zig 0.14.0 already returns
   an optional (`?T`); the `orelse` empty-stack handling is unchanged.

2. Updated the test to record with a real pre-state snapshot and finalise
   with a real post-state snapshot, then assert `ok` for compute_inverse and
   additionally verify_inverse. This exercises the documented contract
   instead of weakening the FFI logic.

The Idris2 ABI (src/interface/abi/Oblibeniser/ABI/Foreign.idr + Types.idr)
remains the source of truth: every `C:<name>` symbol still has a matching
`export fn`, and the Result enum integer values (Ok=0 .. InverseProofFailed=7)
are unchanged and match resultToInt.

Verification:
  - `zig test src/main.zig -lc` -> all 7 tests pass, zero errors/warnings.
  - `idris2 --build oblibeniser-abi.ipkg` -> exit 0.

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

@@ -444,7 +444,7 @@ export fn oblibeniser_undo_pop(handle: ?*Handle) u64 {
         return 0;
     };
 
-    const op_id = h.undo_stack.popOrNull() orelse {
+    const op_id = h.undo_stack.pop() orelse {
         setError("Undo stack empty");
         return 0;
     };
@@ -601,10 +601,24 @@ test "compute and verify inverse" {
     const handle = oblibeniser_init() orelse return error.InitFailed;
     defer oblibeniser_free(handle);
 
-    const op_id = oblibeniser_record_forward(handle, 0, 0);
-    _ = oblibeniser_finalise_forward(handle, op_id, 0);
+    // Record with a real pre-state snapshot, then finalise with a real
+    // post-state snapshot: compute_inverse requires both to be present.
+    var pre = std.mem.zeroes(StateSnapshot);
+    pre.state_hash = 0x1111;
+    var post = std.mem.zeroes(StateSnapshot);
+    post.state_hash = 0x2222;
+
+    const op_id = oblibeniser_record_forward(handle, 0, @intFromPtr(&pre));
+    try std.testing.expect(op_id != 0);
+
+    const finalise = oblibeniser_finalise_forward(handle, op_id, @intFromPtr(&post));
+    try std.testing.expectEqual(Result.ok, finalise);
+
     const compute = oblibeniser_compute_inverse(handle, op_id);
     try std.testing.expectEqual(Result.ok, compute);
+
+    const verify = oblibeniser_verify_inverse(handle, op_id);
+    try std.testing.expectEqual(Result.ok, verify);
 }
 
 test "undo stack push and pop" {