chore(licenses): move api package to SPDX license headers

[david-allison]

Note

Assisted-by: Claude Opus 4.7 - wording + code + tests

Purpose / Description

SPDX is a modern alternative to the full GPL header. In use by the Linux kernel.

SPDX headers are advantageous, as they reduce line noise, makes reviews easier, and makes it impossible for someone to accidentally use GPL-3.0-only, or GPL-3.0-or-later in the API.

I selected the API module for a sample as it's LGPL and limited in scope.

Fixes

• Part of Move to SPDX for file-level license headers (mono-license GPL/LGPL only) #20954

Approach

brew install reuse
reuse download GPL-3.0-or-later
reuse download LGPL-3.0-or-later

• Add lint
• Manually fixup API files

Note: // SPDX-FileCopyrightText: allows dropping the Copyright (c), but I retain the full copyright declarations for existing contributors. The upsides of a few characters aren't worth the downsides

How Has This Been Tested?

Unit tested

Learning (optional, can help others)

https://reuse.software/tutorial/
https://www.kernel.org/doc/html/v5.7/process/license-rules.html
https://spdx.github.io/spdx-spec/v3.0.1/annexes/spdx-license-expressions

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner