fix(experimental): normalize recent GPU data-frame SPDX headers

[ibgreen-openai]

Goals

Restore the repository-wide SPDX source-header test on current master without changing runtime behavior or license ownership.

Changes

• Normalize existing vis.gl copyright comments to SPDX-FileCopyrightText in the recently added experimental batched GPU hash index and luDF sources/tests.
• Preserve the existing MIT license expressions and the existing vis.gl contributors copyright holder verbatim.
• Keep this prerequisite separate from the command-graph, GPU-table, Arrow, and vector-similarity feature pull requests.

Verification

• nvm use (Node 22.22.1).
• yarn install: the configured package registry returns HTTP 403 for required tarballs; verification reuses the existing isolated dependency tree and CI performs the normal immutable install.
• Focused repository-wide SPDX source-header suite.
• yarn lint fix.
• yarn build after final source and formatting changes.
• yarn test-node.
• CI=true yarn test with real Chromium/WebGPU coverage.
• yarn website:build.
• (cd website && yarn build).

Risks

Comment-only SPDX normalization; no runtime, type, API, or ownership changes.