chore: update source file license headers to GPL-2.0-or-later

[digital-michael]

Goal

Update all source file SPDX headers from GPL-2.0 to GPL-2.0-or-later to enable Rust crate compatibility in Phase 5+.

Background

Input Leap is GPL-2.0. SeamGate uses GPL-2.0-or-later (the license file remains GPL-2.0 as the minimum; "or later" is expressed in source headers). This is required so SeamGate can link against GPL-3.0-only Rust crates in future phases.

Acceptance criteria

• All .cpp, .h, .mm files updated: SPDX-License-Identifier: GPL-2.0-or-later
• New files added by SeamGate use GPL-2.0-or-later from the start
• LICENSE file remains unchanged (upstream GPL-2.0 text)
• PR merged to phase/0

Notes

A script approach is fine for the bulk update. Audit after to catch any files that use prose license text instead of SPDX identifiers.