separatorBefore and separatedFirstBlock ask whether a gap holds a + with two different tests

Split from #325, row R7 (verified at main ffdc1860).

The question: does this gap hold a +?

The reader's answer: the recorded GapLine[] on the item block.

The two readers: separatorBefore in src/print/list.ts l.177 (leadingGap.lastIndexOf("+") + 1) and separatedFirstBlock in src/print/list-hazard.ts l.362 (gap.length > 0). Both read the recorded fact, so neither is a text re-derivation; they are two questions phrased differently over one record and can drift.

Remedy recorded on #325 (review finding): one shared gapHoldsAPlus(gap) predicate, not a recorded fact; a proposal to fold this into a tail fact was a category error because ItemBlock.gap stays either way.

Closes when: one predicate, two callers, and a test that the two former tests agreed on every gap the list-shape grid produces (or the witness where they did not, filed).

Related: #325 (the catalog), #90 (the class), #269 (the bodyless description tail, in flight; lands first).

Start with separatorBefore in src/print/list.ts at line 177 and separatedFirstBlock in src/print/list-hazard.ts at line 362, then inspect the list-shape grid tests. Confirm how each caller interprets the recorded GapLine[] and whether their existing tests agree. Done means one shared gap predicate is used by both callers, with coverage for every gap produced by the grid or a filed witness where they differ.