test(core): cover late nested video snapshots

[miguel-heygen]

What

• Add regression coverage for a template host at global 15s with video authored at local 5s.
• Assert that the canonical runtime start resolves to global 20s.
• Verify that both the host and video remain visible during snapshot capture.

Why

A snapshot could hide a later nested video by interpreting its authored start as root-global instead of scene-local. Current main already contains the canonical runtime timing fix; this PR locks the distinct late-host timing shape as regression coverage without adding overlapping production logic.

How

Exercise the runtime visibility resolver with a nested template host and a local-time video, then assert the resolved absolute start and visibility at the affected global timestamp.

Test plan

• Regression fails with the old local-time visibility rule: expected visible, received hidden at global 20s.
• packages/core: bunx vitest run src/runtime/init.test.ts — 69 passed.
• Core typecheck passed.
• Full workspace build passed.
• Changed-file format, lint, diff, and commit hooks passed.
• Fixture snapshot checks at 3.4s and 20.0s passed.
• check --snapshots --at 3.4,20 --json passed runtime, layout, and lint checks.

[miguel-heygen]

Closing as redundant after root-cause triage. The production fix already merged in #2859, and current main already has generalized coverage for later template-mounted scene-local video visibility plus arbitrary-depth absolute media-start resolution. This PR adds the same invariant again with different timestamps (15 + 5 = 20) but no new behavior or boundary. The verified issue remains fixed; we do not need a second test-only PR for equivalent coverage.