chore(playground): remove un-instantiated ffi/zig template subtree (#331)

`echidna-playground/ffi/zig/` is un-instantiated scaffolding: it still
contains literal `{{project}}` / `{{PROJECT}}` placeholders, so it
**cannot compile**. Nothing in the playground references it, and the
real Zig FFI lives at `ffi/zig/` and `src/zig_ffi/`.

This follows the repo's own precedent in `docs/PROOF-NEEDS.md`, where
un-instantiated template scaffolding was removed rather than left to
masquerade as implementation.

Verified: a reference sweep across the playground's JSON, ReScript,
Forth, AsciiDoc, Markdown and HTML files finds no mention of the removed
subtree.

Author: hyperpolymath