fix(experimental): normalize luDF SPDX headers

[ibgreen-openai]

Goals

• Restore the SPDX source-header guard on current master.
• Keep the fix limited to ownership metadata with no runtime or test-semantic changes.

Changes

• Replace the legacy copyright line with the canonical SPDX-FileCopyrightText line in seven luDF/hash-index source and test files introduced by feat(experimental): add immutable luDF dataframe foundation #2933 and feat(experimental): build shared GPU hash indexes from preserved batches #2960.

Verification

• test/examples/spdx-source-headers.node.spec.ts: 6/6 tests passed.
• git diff --check: passed.
• Reviewed the tracked diff: exactly seven one-line header replacements.
• yarn lint fix: attempted, but the reused dependency environment does not provide ocular-lint; the changes are comment-only and preserve the repository's canonical header format.
• nvm use, yarn install, yarn build, yarn test, yarn website:build, and (cd website && yarn build) were not rerun locally; CI is the authoritative full-suite gate for this metadata-only fix.

Risks

• None expected. This changes comments only.