Layout-first readiness says b-roll 'can be added later' even after b-roll is placed

[seekmistar01]

Problem

In the layout-first start (preview/layout-first.html / layout-first.js), the placement-readiness line always ends with "Optional b-roll can be added later." — even after the creator has already placed a b-roll video.

So a creator who drops a b-roll clip into the optional slot still reads that it "can be added later", which contradicts what they just did and gives no acknowledgement that the optional slot is filled.

Why it matters

The readiness line is the single status a creator reads to know what is done and what is left. Telling them b-roll "can be added later" when it is already in place is misleading on the primary, now-gating layout screen.

Proposed fix

Make the optional-b-roll part of the readiness line reflect actual state:

• When the b-roll slot is filled, acknowledge it ("Optional b-roll is in place.").
• When it is empty, keep inviting it ("Optional b-roll can be added later.").

This should update whenever b-roll is placed or removed, in both the "required videos ready" and "still missing required videos" messages. Required-slot gating and the Continue handoff are unchanged (b-roll stays optional and never gates Continue).