P1: fix Zig FFI to compile and match the Idris2 ABI (#39)

## Summary

`src/interface/ffi/src/main.zig` did not compile under Zig 0.14.0. This
PR fixes it with two minimal, FFI-only changes. The Idris2 ABI
(`src/interface/abi/Anvomidaviser/ABI/Foreign.idr` + `Types.idr`) is the
source of truth and is unchanged.

## Fixes

1. **Syntax error (`expected statement, found ';'`, ~line 288):** the
`if (!h.initialized) { ... }` block at the end of
`anvomidaviser_get_string` was terminated with a stray `;`. An `if` used
as a statement takes no trailing semicolon, so the `};` after the block
body was parsed as an empty statement. Removed the spurious semicolon.

2. **`Handle` was a Zig `opaque {}` with fields:** it was declared `pub
const Handle = opaque { allocator, initialized }` but then instantiated
(`handle.* = .{...}`) and field-accessed (`h.allocator`,
`h.initialized`). Zig `opaque` types are unsized and cannot hold or
expose fields, so this could never compile once the parser passed the
syntax error. Changed `Handle` to a concrete `struct`, matching the
house idiom in alloyiser's exemplar (its `Model` struct). It is still
only ever handed to C as a pointer, so it remains opaque across the C
ABI.

## ABI alignment (preserved)

- Every `C:anvomidaviser_*` symbol in `Foreign.idr` has a matching
`export fn` (19/19).
- Result enum integer values match `Types.idr`'s `resultToInt` exactly:
`ok=0, error=1, invalid_param=2, out_of_memory=3, null_pointer=4,
rule_violation=5`.
- No exported symbol names or result codes were changed.

## Verification

- `cd src/interface/ffi && zig test src/main.zig -lc` → all 3 tests
pass, 0 errors/warnings.
- `cd src/interface/abi && idris2 --build anvomidaviser-abi.ipkg` → exit
0 (build dir removed afterward).

Note: any rust-ci / Hypatia / governance red checks are pre-existing
estate-infrastructure 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

@@ -38,12 +38,12 @@ pub const Result = enum(c_int) {
     rule_violation = 5,
 };
 
-/// Library handle (opaque to prevent direct access)
-pub const Handle = opaque {
-    // Internal state hidden from C
+/// Library handle. Opaque to C (passed as a pointer); fields are private to
+/// this module. A concrete struct (not Zig `opaque`) so internal state can be
+/// stored and accessed here.
+pub const Handle = struct {
     allocator: std.mem.Allocator,
     initialized: bool,
-    // Add your fields here
 };
 
 //==============================================================================
@@ -285,7 +285,7 @@ export fn anvomidaviser_get_string(handle: ?*Handle) ?[*:0]const u8 {
     if (!h.initialized) {
         setError("Handle not initialized");
         return null;
-    };
+    }
 
     const result = h.allocator.dupeZ(u8, "Anvomidaviser score pending") catch {
         setError("Failed to allocate string");