The canonical fork-id safelist, shared by every fork-serving backend. A
fork id is caller-chosen and a backend that overlays a fork inlines it into
a copy-on-write query as a quoted identifier, so the charset must be a
strict safelist, not just a length bound: this is the one anti-injection
rule. A valid id is non-empty, at most MAX_FORK_ID_BYTES bytes, and made
only of ASCII letters, digits, -, _, and .. A stream-scoped id
stream:<stream>/<local> is valid when the whole id fits the same bound and
both parts pass the same charset rule. A backend that binds the id as a
parameter may skip the call, but one that inlines it gets the defense for
free by calling this before use.