MINOR from the 2026-04-22 review (build.rs:47-64): the guard used
strip_prefix("connect-src") to find the directive, which would also
match hypothetical future directives like connect-src-elem and
validate the wrong allow-list.
Switches to split_once(char::is_whitespace) on each directive and
requires the first token to equal "connect-src" exactly. Robust
against prefix collisions with any future CSP3 directive
additions.