M6-03: Add demo gallery with redistributable files

[DrunkOnJava]

Created from the roadmap audit in TODO.md.

Source: https://github.com/DrunkOnJava/rvt-rs/blob/main/TODO.md#L800
Tracking commit: 1ec8105

Scope

Labels: priority:P1, type:feature, area:viewer, area:corpus

• Implement demo loading from docs/viewer-demos.json or successor.
• Only include redistributable files.
• Add thumbnails, expected decoded counts, and export quality labels.

Acceptance criteria:

• Viewer can load at least one demo without local file selection.
• Demo metadata includes license/provenance.
• Network behavior still complies with privacy posture.

Definition of done

• The TODO acceptance criteria for this section are implemented or explicitly superseded by a linked decision.
• Tests, fixtures, or documentation are added at the level appropriate to the change risk.
• User-facing behavior and limitations remain honest in README/docs/viewer messaging.
• Relevant CI checks pass before the issue is closed.