P1: fix Zig FFI to build and match the Idris2 ABI (#37)

## Summary

The Zig FFI (`src/interface/ffi/src/main.zig`) failed to compile under
**Zig 0.14.0**. This PR fixes the two compile errors with minimal,
behaviour-preserving changes. Only `src/interface/ffi/src/main.zig` is
touched; the **Idris2 ABI is the source of truth** and was left
untouched.

## Errors (from `zig test src/main.zig -lc`)

1. `src/main.zig:118` — `@bitCast size mismatch: destination type 'u64'
has 64 bits but source type 'i128' has 128 bits`. In `betlangiser_init`,
`std.time.nanoTimestamp()` returns `i128`, but it was `@bitCast`
directly into the `u64` PRNG seed.
2. `src/main.zig:558` — `name shadows primitive 'u1'`. In
`sampleDistribution` (Box-Muller normal sampling), the locals `u1`/`u2`
shadow the built-in primitive type `u1`, which is an error in Zig 0.14.

## Fixes

1. Reinterpret the `i128` timestamp as `u128`, then truncate to `u64`:
   `@truncate(@as(u128, @bitcast(std.time.nanoTimestamp())))`.
2. Rename the Box-Muller locals `u1`/`u2` → `unif1`/`unif2`.

Net diff: 4 insertions, 4 deletions in one file.

## ABI fidelity (preserved)

No exported C symbol names, signatures, or integer encodings changed:

- All 26 `C:betlangiser_*` symbols in `Foreign.idr` still map to
matching `export fn`s.
- `Result` enum `ok=0 … sampling_failed=6` matches `Types.idr`
`resultToInt`.
- `TernaryBool` encoding `t_false=0, t_true=1, t_unknown=2` matches
`ternaryToInt`.
- `DistributionTag` encoding `normal=0 … custom=4` matches
`distributionTag`.

## Verification

- `cd src/interface/ffi && zig test src/main.zig -lc` → **all 9 tests
pass**, zero errors/warnings, exit 0.
- `cd src/interface/abi && idris2 --build betlangiser-abi.ipkg` → **exit
0** (build dir removed afterwards).
- Symbol-coverage check: every `C:<name>` in `Foreign.idr` has a
matching `export fn <name>` in `main.zig` (26/26 OK).

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

@@ -115,7 +115,7 @@ export fn betlangiser_init() ?*anyopaque {
     engine.* = .{
         .allocator = allocator,
         .initialized = true,
-        .prng = std.Random.DefaultPrng.init(@bitCast(std.time.nanoTimestamp())),
+        .prng = std.Random.DefaultPrng.init(@truncate(@as(u128, @bitCast(std.time.nanoTimestamp())))),
         .distributions = std.ArrayList(*Distribution).init(allocator),
     };
 
@@ -555,9 +555,9 @@ fn sampleDistribution(engine: *Engine, dist: *const Distribution) f64 {
     return switch (dist.tag) {
         .normal => blk: {
             // Box-Muller transform for normal sampling
-            const u1 = random.float(f64);
-            const u2 = random.float(f64);
-            const z = @sqrt(-2.0 * @log(u1)) * @cos(2.0 * std.math.pi * u2);
+            const unif1 = random.float(f64);
+            const unif2 = random.float(f64);
+            const z = @sqrt(-2.0 * @log(unif1)) * @cos(2.0 * std.math.pi * unif2);
             break :blk dist.param1 + dist.param2 * z;
         },
         .uniform => dist.param1 + (dist.param2 - dist.param1) * random.float(f64),