StageName - temper

A stage's name, and — because [StageName::parse] is the only constructor — a proof that the
name is a safe SQL identifier.

The compiler (beat C, Task 9) emits stage names as CTE identifiers. Parse-don't-validate is the
whole design: a name that cannot be constructed cannot reach SQL, so there is deliberately no
new_unchecked. The accepted shape is [a-z][a-z0-9_]{0,62} — a leading lowercase letter,
then up to 62 more lowercase-alphanumeric-or-underscore characters (63 total).