the len(rs) != 1 guard in scriptioContinuaOnly is unreachable as called so no fixture can enter it

the len(rs) != 1 guard in scriptioContinuaOnly is unreachable as called so no fixture can enter it

Reported by the round-5 builder and left in place with the reasoning stated in 6330b8b2's body rather than claimed as tested, which was the right call at the time.

textUnits emits a scriptio-continua letter as its own single-rune unit, so a multi-rune unit never begins with one, and the sibling test !isScriptioContinuaLetter(rs[0]) already rejects every shape len(rs) != 1 would. Deleting it leaves the whole suite green, and the builder found no text reaching ValidateText that distinguishes the two.

The first reading of this record routed it to deletion, on the sibling precedent: itd-189's round 3 deleted two stand-downs under the finding that a branch no fixture can enter is how a guard comes to look tested, and iss-2608301519254240 was raised against the twin it left behind.

Inspection reversed that, and the difference is worth keeping. Those stand-downs were defensive scaffolding against states their callers could not produce. This one is half of what the predicate MEANS. "Every unit is a single scriptio-continua letter" and "every unit begins with one" are different claims which agree only because of how textUnits splits today, and deleting the length test would leave the weaker claim in the code, agreeing silently for as long as that holds and no longer.

So it is pinned rather than removed, at the helper, which is the only level a fixture for it exists at. Unreachable through ValidateText is not the same as unreachable, and the test that proves it is load-bearing is the one that kills the mutant textUnits cannot currently feed it.

Grounds

• pursued: We expect a direct helper test to be the right close rather than deletion, because the guard states what a unit must be and not merely what the caller happens to send, so removing it would leave a weaker claim agreeing silently with the stronger one until textUnits changed.