P1: fix Zig FFI to compile under Zig 0.14.0 (#166)

## Summary

The Zig FFI layer (`src/interface/ffi/src/main.zig`) did not compile
under Zig 0.14.0. This fixes the two reported errors (and confirms there
are no further ones), with no change to the C ABI surface.

## Errors and fixes

1. **`extern struct cannot contain a field of type 'mem.Allocator'`**
(was at line ~94)
- `Handle` and `DbConnection` were declared as `extern struct` while
holding a `std.mem.Allocator` field. Zig 0.14 rejects this because
`Allocator` is not extern-ABI-compatible.
- These handles only ever cross the C boundary as opaque `?*T` pointers,
so their layout is private to this module and `extern` was never needed.
Both changed to a normal `struct` (the house idiom — same as the
alloyiser exemplar, which uses a normal `struct` passed as an opaque
pointer).
- All `export fn`s already take/return them as `?*Handle` /
`?*DbConnection`, so the C ABI is unchanged.

2. **`pointless discard of function parameter`** (was at lines ~470–472)
- `verisimiser_vql_free_result` both used `result_ptr` (`if (result_ptr
== 0) return;`) and then redundantly discarded it (`_ = result_ptr;`).
Zig 0.14 errors on discarding a parameter that is also used. Removed the
redundant discard, keeping the real use.

No further errors surfaced after these two fixes.

## ABI preserved (source of truth)

The Idris2 ABI (`src/interface/abi/Verisimiser/ABI/Foreign.idr` +
`Types.idr`) is the source of truth and is unchanged:
- All 24 exported C symbol names preserved 1:1 (every `C:<name>` in
`Foreign.idr` has a matching `export fn <name>`).
- `Result` enum integer values still match `resultToInt`: `Ok=0,
Error=1, InvalidParam=2, OutOfMemory=3, NullPointer=4,
ConnectionFailed=5, ChainCorrupted=6, SidecarUnavailable=7`.

Only `src/interface/ffi/` was touched.

## Verification

- `cd src/interface/ffi && zig test src/main.zig -lc` → all 8 tests
pass, 0 errors/warnings
- `zig build-obj src/main.zig -lc` → exit 0 (clean C-ABI object)
- `cd src/interface/abi && idris2 --build verisimiser-abi.ipkg` → exit 0
- Symbol cross-check: every `C:<name>` in `Foreign.idr` maps to an
`export fn` (empty diff)

> Note: any rust-ci / Hypatia / governance red checks are pre-existing
estate-infra and 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

@@ -90,7 +90,12 @@ pub const DimensionMask = u32;
 
 /// VeriSimiser library handle (opaque to C callers).
 /// Holds the augmentation state, sidecar connections, and configuration.
-pub const Handle = extern struct {
+///
+/// This is a normal Zig struct, never an `extern struct`: it holds a
+/// `std.mem.Allocator` (which is not extern-compatible) and is only ever
+/// crossed the C boundary as an opaque `?*Handle` pointer, so its layout is
+/// private to this module.
+pub const Handle = struct {
     allocator: std.mem.Allocator,
     initialized: bool,
     backend: DatabaseBackend,
@@ -102,7 +107,9 @@ pub const Handle = extern struct {
 };
 
 /// Database connection handle (opaque to C callers).
-pub const DbConnection = extern struct {
+/// Normal struct for the same reason as `Handle`: it holds an allocator and
+/// only crosses the C boundary as an opaque pointer.
+pub const DbConnection = struct {
     allocator: std.mem.Allocator,
     backend: DatabaseBackend,
     connected: bool,
@@ -469,7 +476,6 @@ export fn verisimiser_vql_query(
 export fn verisimiser_vql_free_result(result_ptr: u64) void {
     if (result_ptr == 0) return;
     // TODO: free result set memory
-    _ = result_ptr;
 }
 
 //==============================================================================