Postbuild script computes SHA-256 of every inline <script> in
build/index.html and rewrites build/_headers — replacing the
script-src 'unsafe-inline' relaxation with the matching hashes. The
hash regenerates per build (SvelteKit bootstrap embeds a per-build
registration call) so the script must run on every build; chain it
into both `npm run build` and `build:gh`.
verify-build extended to assert build/_headers script-src no longer
contains 'unsafe-inline', so the inject step's output is enforced in
CI. style-src 'unsafe-inline' stays — Svelte's `style:` directives
emit inline attributes that hashes can't cover.