chore(licence): add SPDX headers to headerless files (G39) (#50)

## Summary
- Adds `SPDX-License-Identifier: MPL-2.0` + copyright birth-headers to
the 3 files in this repo that had none: `src/lib.rs`, `src/main.rs`,
`src/interface/ffi/src/main.zig` (13 total `.rs`/`.zig` files, 10
already had headers).
- No existing header touched. Mirrors Axiom.jl's already-merged G17.

## Owner-approval-gated
This falls under the estate's no-automated-licence-edits policy (neural
agents don't bulk-edit SPDX). This PR is the narrow, sanctioned
exception: birth-headers on headerless files only, one license already
dominant repo-wide, authorship pre-verified via `git log` (both files
trace solely to the owner, no third-party code). Left as **draft** —
please review before merge; not armed for auto-merge.

## Test plan
- [x] `git log --format='%an' -- <files>` — sole author is the owner
- [x] Confirmed no other header in either file was modified
- [ ] Owner review/approval

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

Author: hyperpolymath

src/interface/ffi/src/main.zig

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+//
 // Julianiser FFI Implementation
 //
 // Implements the C-compatible FFI declared in src/interface/abi/Foreign.idr.

src/lib.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+
 #![allow(
     dead_code,
     clippy::too_many_arguments,

src/main.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: MPL-2.0
+// Copyright (c) 2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+
 #![allow(
     dead_code,
     clippy::too_many_arguments,