P1: fix Zig FFI null-handle test under Zig 0.14.0 (#31)

## The bug

`zig test src/main.zig -lc` failed on `test "error handling — null
handle"` (exit code 1). The test invokes `bqniser_type(null, 0)` and
then asserts `bqniser_last_error() != null` — i.e. that calling an
accessor with a null handle records the null-pointer error condition,
consistent with the ABI's `NullPointer` result code (`NullPointer = 4`
in `src/interface/abi/Bqniser/ABI/Types.idr`).

The function was the bug, not the test. `bqniser_type`'s null-handle and
uninitialized-handle paths returned `0` silently:

```zig
const h = castHandle(handle) orelse return 0;   // no setError -> last_error stays null
if (!h.initialized) return 0;
```

So `last_error` was never populated and the assertion failed. (The prior
`lifecycle` test ends in `bqniser_free`, which calls `clearError()`, so
`last_error` is null entering the null-handle test.)

## The fix

On the null/uninitialized paths of the accessor functions that
previously swallowed the error (`bqniser_type`, `bqniser_bound`,
`bqniser_to_f64`), call `setError(...)` to record the condition — the
same house idiom already used by `bqniser_eval`, `bqniser_call1`,
`bqniser_call2`, and `bqniser_read_f64_arr`, and matching the worked
exemplar in `alloyiser` where the null-handle path maps to the ABI's
null-pointer condition. This makes the null-handle path observably
report the error, consistent with the ABI's `NullPointer` semantics.

The test's expectation was correct and is unchanged; only the
implementation was corrected.

## ABI preserved

- No exported C symbol names changed; every `C:<name>` in `Foreign.idr`
still has a matching `export fn <name>`.
- The `Result` enum integer values are unchanged and still match
`resultToInt`: `ok=0, error=1, invalid_param=2, out_of_memory=3,
null_pointer=4, eval_error=5`.
- Only `src/interface/ffi/src/main.zig` was touched.

## Verification

- `cd src/interface/ffi && zig test src/main.zig -lc` → **All 4 tests
passed** (zero errors/warnings).
- `cd src/interface/abi && idris2 --build bqniser-abi.ipkg` → **exit 0**
(build dir removed afterwards).
- ABI symbol ↔ Zig export parity confirmed; Result values match
`resultToInt`.

Note: any rust-ci / Hypatia / governance red checks are pre-existing
estate-infra issues unrelated to this Zig-only change.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

---
_Generated by [Claude
Code](https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH)_

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

Author: hyperpolymath

src/interface/ffi/src/main.zig

@@ -191,17 +191,29 @@ export fn bqniser_call2(handle: ?*anyopaque, f: BQNV, w: BQNV, x: BQNV) BQNV {
 
 /// Get the type tag of a CBQN value.
 export fn bqniser_type(handle: ?*anyopaque, val: BQNV) u32 {
-    const h = castHandle(handle) orelse return 0;
-    if (!h.initialized) return 0;
+    const h = castHandle(handle) orelse {
+        setError("Null handle in bqniser_type");
+        return 0;
+    };
+    if (!h.initialized) {
+        setError("Handle not initialized");
+        return 0;
+    }
     _ = val;
     // TODO: return @intCast(bqn_type(val))
     return 0;
 }
 
 /// Get the element count (bound) of a BQN array.
 export fn bqniser_bound(handle: ?*anyopaque, val: BQNV) u64 {
-    const h = castHandle(handle) orelse return 0;
-    if (!h.initialized) return 0;
+    const h = castHandle(handle) orelse {
+        setError("Null handle in bqniser_bound");
+        return 0;
+    };
+    if (!h.initialized) {
+        setError("Handle not initialized");
+        return 0;
+    }
     _ = val;
     // TODO: return bqn_bound(val)
     return 0;
@@ -230,8 +242,14 @@ export fn bqniser_read_f64_arr(handle: ?*anyopaque, val: BQNV, buf: BQNV) Result
 
 /// Extract a scalar f64 from a BQN numeric value.
 export fn bqniser_to_f64(handle: ?*anyopaque, val: BQNV) f64 {
-    const h = castHandle(handle) orelse return 0.0;
-    if (!h.initialized) return 0.0;
+    const h = castHandle(handle) orelse {
+        setError("Null handle in bqniser_to_f64");
+        return 0.0;
+    };
+    if (!h.initialized) {
+        setError("Handle not initialized");
+        return 0.0;
+    }
     _ = val;
     // TODO: return bqn_toF64(val)
     return 0.0;