Issue: C Position Vector Validation 2026-08-20

issue025 [ ] C direct API accepts invalid break positions

Status: Closed by direct contract verification on 2026-08-20.
Summary: The C boundary validates vector shape and signed integer type, but not the exclusive break-index range, monotonic ordering, or duplicate policy of hyphen/forbidden positions.
Expected vs Actual: Malformed direct arguments should signal ekp-c-invalid-input per the C API contract. Current [-1] and [99] vectors return a layout result instead.
Related: ekp_c/ekp.c, ekp_c/ekp_kp.c, task040.
Fix: Shared preflight now rejects out-of-range, duplicate, and non-increasing vectors for both direct and batch calls.
Verification: Direct/batch C ERT 15/15, portable warning-clean build, 300-case fuzz, and Elisp parity pass.
Resolved At: 2026-08-20.
Resolved By: Developer implementation and contract verification.
Commit: Pending hardening commit.