finding(plugin-timeline): a `null` gantt BAR (`items: [{ items: [null] }]`) is refused through `malformedDate` as "startDate is undefined, which is not a valid date" — the wrong fault, one level below #7164

[os-litant]

Found while implementing #7164. Not repaired there: the ruling named the ROW and row.items shapes (a row is an object; its items, when present, is an array) and the bars inside a row stay z.any() in the mirror. A bar that is not a bar is one level below that scope.

What is wrong

Measured in render through the real TimelineRenderer, on a67abdc88 (main) and on the #7164 branch at 8e1dc8274 — identical:

items: [{ label: 'R', items: [null] }]
 -> NAMED Unusable gantt date range — items[0].items[0].startDate is undefined, which is not a valid date. …

The outcome is defined (no crash — findUnusableGanttDate reads rowItems[0]?.startDate, gets undefined, and refuses), but the copy names the WRONG fault: the bar is null, not an item whose startDate is missing. This is the same mis-naming class #7164 fixed for rows ("items[0] is null, which is not a valid date" named the wrong fault for a row that is not a row), one level down.

And validate accepts it: TimelineSchema.items[].items is z.array(z.any()), so a null bar is green at authoring time.

Options

• A — both doors, as finding(plugin-timeline): a gantt row that is null, or whose items is a truthy non-array, crashes the render — findUnusableGanttDate reads the walk defensively and calculateDateRange reads it bare #7164 did for rows: the mirror declares a bar as an object (z.array(z.object({}).passthrough()) for row.items — a contract change, Clause ②), and classifyGanttRows names items[0].items[0] is null at render-time. The existing malformedRow copy says "which is not a row shape", which reads slightly off for a bar; either a fourth path level in the same sentence or a small copy adjustment across the ten packs would be needed, and that is a ruling.
• B — mirror only: refuse the null bar at authoring; leave the render-time copy as the defined outcome it is today.
• C — leave it: it is a defined, non-crashing outcome.

Recommendation: at least B — an authored null bar is an error an AI-authored document should be told about at validate, not by a date diagnostic naming a key it never wrote.

Blocked-by: #7164 (the mirror's row declaration and classifyGanttRows both land there).