feat(studio): let an embedder draw the preview edge to edge with fillBox - #4408

What

NLEPreview gets an optional fillBox prop. When it is on, the picture is drawn edge to edge in the preview's box instead of 8px in from each side on a dark band. Without the prop nothing changes.

Why

An app that embeds NLEPreview and sizes its box to the film's shape saw the film smaller than the box, framed by a black band. In a 640x360 box with a 16:9 film, the picture was 611.5x344. There was no way to turn the band off: the inset lived in two places (the inner container's inset-2 and a 16px subtraction in resolvePreviewStageSize), and neither was reachable from props.

How

• One constant owns the per-side inset (PREVIEW_STAGE_INSET_PX = 8). fillBox sets it to 0.
• The same value drives the inner container's inset and the stage-size math, so the layout and the sizing cannot drift apart.
• resolvePreviewStageSize takes the inset as a parameter that defaults to the constant, so existing callers keep their numbers.
• The viewport's p-2 is removed: every child of the viewport is absolutely positioned, so the padding never affected layout.

Test plan

• New test in NLEPreview.test.ts: in a 640x360 box, the default stays 611.5556x344 with an 8px inset, and fillBox gives a 640x360 stage with no inset. It passes on this branch (11 passed, exit 0). It fails when fillBox is ignored (exit 1, only this test red).
• tsc --noEmit on packages/studio, oxlint and oxfmt --check on the changed files: clean.
• A page that mounts NLEPreview in a 640x360 box (pink outline) with a 16:9 registry example, same URL both times. Measured in the browser: main draws 611.5x344 inside the box; this branch with fillBox draws 640x360. With the prop off, this branch's screenshot is pixel-identical to main.

Before

main: the picture sits 8px in from the box on a black band.

After

this branch with fillBox: the picture fills the box.

Review

Independent adversarial review of this head: blocking none, should fix none. Notes: the new test covers a same-shape landscape box without the ruler gutter; the gutter and letterbox paths reuse the same arithmetic (checked by hand, 2 * (inset + gutter) equals the old single subtraction when the inset is 8). Every consumer of the stage geometry (pan and zoom clamp, ruler, deselect outside the frame, drop coordinates) reads live rects, so none assumes the inset.