test(core): cover video timing StaticGuard regression

[phucsd1]

Summary

• extend the existing compiler-derived media timing regression with a canonical <video> element
• verify data-start="6" data-duration="8" is compiled to data-end="14"
• verify StaticGuard accepts the compiled video contract without the misleading source-authoring warning

Why

Issue #2662 originally demonstrated the regression with audio. An A/B investigation of a real video composition found the same failure mode for <video>: v0.7.64 emitted data-end during compilation and StaticGuard then rejected the compiled result for not retaining data-duration. Releases v0.7.65 through v0.7.68 no longer reproduce it.

This PR adds video coverage to prevent that path from regressing. It is test-only and does not change runtime behavior.

Investigation details: #2662 (comment)

Verification

• RED: the added video case fails on v0.7.64 with [StaticGuard] Invalid HyperFrame contract: <video id="demo"> uses data-end without data-duration.
• GREEN: packages/core/src/compiler/htmlBundler.test.ts passes 44/44 on current main
• oxfmt and oxlint pass for the changed file
• TypeScript checks pass for packages/core and packages/studio
• Fallow audit reports no issues in the changed file
• large-file and tracked-artifact checks pass