Trace: stop storing each action screenshot twice #495

Part of #494.

Problem

Every per-action screenshot is stored twice in a trace zip. It is written once as a file under resources/, and again as base64 inside the frame-snapshot event in trace.trace: buildImageFrameSnapshots in packages/trace/src/trace-frame-snapshots.ts builds an HTML document around <img src="data:image/...;base64,..."> and writes resourceOverrides: [].

Measured on the WDIO cucumber example's trace (1.63 MB zip):

• trace.trace is 538 KB zipped, of which 529 KB is the 12 frame-snapshot events.
• Each event is 215 bytes of HTML around a 77 KB base64 copy of a screenshot already in resources/.
• Base64 is a third larger than the image and barely compresses (765 KB raw to 529 KB deflated).

Proposal

Reference the stored resource instead of inlining it: give the <img> a URL and map it to the resource's sha1 through resourceOverrides, which the format already defines for this.

The frame-snapshot events exist for other viewers that read the format (Playwright's trace viewer, Allure's embedded viewer); our player replays from trace.mutations. So the change must be verified in both of those viewers, not only ours.

Expected saving: about a third of a typical zip, with nothing lost.