Apply BSL source headers and enable header check

[justin808]

Context

PR shakacode/shakaperf-old#185 adds the repeatable command for applying BSL source headers, but intentionally does not commit the generated header diff. That keeps the command reviewable and avoids creating conflicts with work in progress.

Follow-up Work

• Merge shakacode/shakaperf-old#184 first so the BSL licensing baseline is in place.
• Merge shakacode/shakaperf-old#185 if the header command is accepted.
• Run ruby scripts/update-license-headers.rb --dry-run and review the selected file list.
• Run ruby scripts/update-license-headers.rb to apply headers in a dedicated generated-header PR.
• Confirm packages/shaka-shared remains untouched because it stays MIT-licensed.
• Confirm generated/vendor/toolchain/snapshot paths remain skipped.
• Add a top-level license:headers:check package script only after the checked-in tree is header-complete.
• Consider wiring that check into CI after the source headers land.

Acceptance Criteria

• Source-header PR contains only the generated header changes plus any needed check-script/CI wiring.
• ruby scripts/update-license-headers.rb --check passes on a clean checkout.
• yarn license:headers:check exists only once it passes on the checked-in tree.
• No files under packages/shaka-shared receive BSL headers.