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

[hyperpolymath]

Summary

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. This PR fixes both, keeping the Idris2 ABI (src/interface/abi/Oblibeniser/ABI/Foreign.idr + Types.idr) as the source of truth.

Errors found

1. Compile error — array_list.ArrayListAligned no longer exposes popOrNull in Zig 0.14.0. oblibeniser_undo_pop called h.undo_stack.popOrNull():

   src/main.zig:447:31: error: no field or member function named 'popOrNull' in 'array_list.ArrayListAligned(u64,null)'
   

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 (its contract requires both pre- and post-state snapshots). The test asserted ok:

   3/7 main.test.compute and verify inverse...expected main.Result.ok, found main.Result.not_reversible
   

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.

No exported C symbol names or Result enum integer values were changed.

ABI conformance

• Every C:<name> in Foreign.idr has a matching export fn <name> (20/20 symbols, verified by diff — no missing/extra symbols).
• Result enum values match resultToInt: Ok=0, Error=1, InvalidParam=2, OutOfMemory=3, NullPointer=4, NotReversible=5, AuditViolation=6, InverseProofFailed=7.

Verification

• cd src/interface/ffi && zig test src/main.zig -lc → all 7 tests pass, zero errors/warnings.
• cd src/interface/abi && idris2 --build oblibeniser-abi.ipkg → exit 0 (build dir removed afterwards).

Only src/interface/ffi/src/main.zig is touched.

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

🤖 Generated with Claude Code

https://claude.ai/code/session_019xMKB3T4Vo5FYC7Czx3JSH

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