chore(license): add SPDX headers + crates.io fallback comment

Adds the required SPDX-License-Identifier headers to the four
Rust source roots. Per estate licence policy these need MPL-2.0
on crates.io (PMPL-1.0-or-later is the preferred form but is not
yet OSI-approved, so platform registries get the explicit
fallback identifier with a comment noting the preference).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

ipv6-only/crates/core/src/lib.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// (PMPL-1.0-or-later preferred; MPL-2.0 required for crates.io)
 //! IPv6-Only Core — High-Assurance Network Types.
 //!
 //! This crate provides the foundational primitives for IPv6 manipulation. 

ipv6-only/crates/subnet/src/lib.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// (PMPL-1.0-or-later preferred; MPL-2.0 required for crates.io)
 //! IPv6 Subnet Calculator — Network Planning Engine.
 //!
 //! This crate provides high-level utilities for partitioning IPv6 address 

ipv6-only/crates/utils/src/lib.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// (PMPL-1.0-or-later preferred; MPL-2.0 required for crates.io)
 //! IPv6 Utilities — High-Assurance Address Manipulation.
 //!
 //! This crate implements the core numerical logic for the IPv6-Only project. 

ipv6-only/fuzz/fuzz_targets/fuzz_main.rs

@@ -1,3 +1,5 @@
+// SPDX-License-Identifier: MPL-2.0
+// (PMPL-1.0-or-later preferred; MPL-2.0 required for crates.io)
 #![no_main]
 use libfuzzer_sys::fuzz_target;