ledmatrix-stocks: the safety harness never renders any content for this plugin

[ChuckBuilds]

What

check_plugin.py reports 8/8 PASS for ledmatrix-stocks, but every one carries the same warning:

[PASS]   64x32  stocks (empty warn: drew nothing but display() returned None, so the mode is not skipped)
[PASS]  128x32  stocks (empty warn: drew nothing ...)
[PASS]   64x64  stocks (fill warn: extent 0%x0%) (empty warn: drew nothing ...)
...

Every panel is blank, so the crash and overflow checks are running against an empty frame. Nothing about the actual ticker layout — fonts, colours, the inline chart, symbol widths — is being tested.

Why

Two reasons compound:

1. No test/harness.json. The plugin ships none, so the harness has no quote data and self.stock_data stays empty.
2. Frame 0 of a scroll is blank. Even with data, the ticker starts off-panel and slides in, so a single display() call renders nothing. I hit this while making the README screenshots — my first render was empty, and only advancing ~60 frames brought the text into view.

Suggested fix

A test/harness.json with a small recorded quote would fix (1). For (2), the harness would need to call display() more than once for scrolling plugins — the documentation renderer added exactly that in #369 as a frames option, and the same idea would apply here.

Both are worth having: this plugin has an inline chart renderer, five font choices and eight colour keys, none of which the harness currently sees.

I have a working fixture in docs/assets/ledmatrix-stocks/mock.json from #394 that could be reused as the harness fixture if that is the direction you want.

Scope

Likely affects any other scrolling plugin without harness data — worth a sweep once the approach is settled.