feat(ffi): Zig FFI consumer demo + CI lane (#6) (#23)

De-templates ffi/zig from the {{PROJECT}} skeleton and proves the C-ABI
contract with a linking consumer.

- build.zig: ported to the Zig 0.15.x module API (b.addLibrary /
  b.createModule), dropped the dead header/bench/docs/integration steps
  that referenced nonexistent files, link_libc (the FFI uses
  std.heap.c_allocator), and added `test` / `consumer` / `check` steps.
- src/main.zig: added the OctadDimension + ProvenanceEntry extern
  structs and verisimdb_data_{octad,provenance}_{encode,decode} with a
  lossless wire format. Fixed two latent template bugs that blocked any
  build: `Handle` was `opaque` *with fields* (now a struct; still
  opaque to C behind ?*Handle), and `callconv(.C)` → `callconv(.c)`
  (0.15 rename). Added 5 round-trip/negative unit tests.
- test/octad_consumer.zig: NEW standalone executable that
  linkLibrary()s the FFI and round-trips both structs across the
  boundary (incl. a short-buffer negative path); exits non-zero on any
  mismatch. Replaces test/integration_test.zig, which was an
  uncompilable {{project}} template stub.
- .github/workflows/zig-ffi.yml: CI lane; Zig pinned by tarball
  sha256 (no third-party action — Scorecard pinned-deps clean) running
  `zig build check`.

Local: `zig build check` → 7/7 unit tests pass; consumer prints
"PASS: verisimdb-data FFI v0.1.0 — OctadDimension + ProvenanceEntry
round-trip OK".

Closes #6.

Co-authored-by: hyperpolymath <hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath