Function invariant

invariant(
condition: unknown,
message: string | (() => string),
): asserts condition
Asserts a condition that can only be false because of a bug, and narrows it for the code after it.

Expected failures return Either; invariant is for the other kind — "upsert returned no row", "step must be non-negative", "AUTH_PROVIDER is required in production". It covers one statement, so a function can mix bug checks with Either-returning failure paths, and functype/prefer-either never sees a throw to report.
```
invariant(row, "upsert returned no row")          // row: Row from here on
invariant(teamDomain && aud, "requires CF_ACCESS_TEAM_DOMAIN and CF_ACCESS_AUD")
invariant(n >= 0, () => `step must be non-negative, got ${n}`)  // message built only on failure
```
Parameters
- condition: unknown
  Must be truthy. Narrowed to truthy after the call.
- message: string | (() => string)
  The error message, or a thunk that builds it only when the check fails.
Returns asserts condition

Throws
InvariantViolation when condition is falsy.