chore(licence+hygiene): SPDX headers on headerless estate .idr/.zig (G17) + untrack .zig-cache (#57)

## What

**G17** — add `SPDX-License-Identifier: MPL-2.0` to the seven
estate-authored `.idr`/`.zig` source files that had **no** SPDX header,
plus a small build-cache hygiene fix.

## Licence-guardrail compliance

Made with **explicit owner approval (2026-07-01)**, per the
manual/owner-only licence-edit mandate:
- Only files that had **no** SPDX header were touched — **no existing
identifier modified**, no bulk sweep.
- All seven are estate-authored MPL-2.0 (the repo's classification);
none are vendored/third-party.
- Header comment style matches each language's existing convention (`--`
for Idris, `//` for Zig).

Files:
- `packages/SMTLib.jl/src/abi/{Types,Layout,Foreign}.idr`
- `ffi/idris/{hello,main}.idr`
- `ffi/zig/src/main.zig`, `zig/build.zig`

## Hygiene

`.gitignore` covered `zig/.zig-cache/` but missed `ffi/zig/.zig-cache/`,
so Zig build-cache artifacts had been committed. Added the missing
ignore rule and untracked the cache (files kept on disk, not deleted).

## Verification

- Re-scan: every tracked estate `.idr`/`.zig` source now carries SPDX.
- No Julia code touched — `.idr`/`.zig` are FFI/ABI files not compiled
by the Julia test suite, so `Pkg.test` is unaffected. The one-line
comment prepends are syntactically valid in both languages.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---
_Generated by [Claude
Code](https://claude.ai/code/session_01UPFC9YQ7g9gc3VnRox42Q1)_

Co-authored-by: Claude <noreply@anthropic.com>

Author: hyperpolymath

.gitignore

@@ -104,3 +104,4 @@ deps/
 build/
 dist/
 docs/Manifest.toml
+ffi/zig/.zig-cache/

ffi/idris/hello.idr

@@ -1,3 +1,4 @@
+-- SPDX-License-Identifier: MPL-2.0
 module Main
 
 main : IO ()

ffi/idris/main.idr

@@ -1,3 +1,4 @@
+-- SPDX-License-Identifier: MPL-2.0
 module Main
 
 import System.FFI