feat(cli): add Zig CLI consumer (subprocess architecture)

Adds the verisim standalone CLI — the agreed-upon CLI-first consumer target,
chosen for OS process isolation (dependability + security over priority order).

Architecture:
- src/Abi/VCLProtocol.idr  — Idris2 formal ABI for the stdio wire protocol
  (VCLQuery, VerdictResult, WellFormedExchange + A2ML framing constants)
- src/vcl_server.jl         — Julia server script; spawned as child process,
  reads VCL from stdin, proves via Verisim package, emits A2ML [vcl-verdict]
- ffi/zig/src/verisim_cli.zig — Zig binary; resolves package path, spawns
  Julia, pipes queries/verdicts (4/4 unit tests: Pass, ParseError, empty, multi)
- ffi/zig/build.zig + build.zig.zon — Zig 0.15 build config
- justfile: cli-build, cli-build-release, cli-test, cli-query recipes

No libjulia embedding — subprocess boundary is the isolation mechanism.
VERISIM_PACKAGE_PATH env var overrides package location; fallback: ../../../..
relative to binary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: hyperpolymath

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