[alpha.5] T6.3 Test attribute / event / slot / form / theming for third-party WCs

[SisyphusZheng]

Parent\n\n#134\n\n## What to build\n\nAdd automated or smoke tests for third-party WC behavior in the compatibility harness.\n\n## Acceptance criteria\n\n- [ ] Attribute/property round-trip tested.\n- [ ] Composed event handling tested.\n- [ ] Slot composition tested.\n- [ ] Form-associated behavior tested if the external WC supports it.\n- [ ] Theming/token interop tested.\n\n## Blocked by\n\n- #175

[SisyphusZheng]

Closing as completed or superseded by the alpha.5 train.

Evidence:

• PR feat(alpha.5): SPA mode + client router + desktop proof #121 (feat(alpha.5): SPA mode + client router + desktop proof) was merged and its Alpha.5 Issue Coverage table marks the alpha.5 items done/superseded and lists them for closure: feat(alpha.5): SPA mode + client router + desktop proof #121
• v0.41.0-alpha.5 was published as a GitHub prerelease: https://github.com/open-element/openelement/releases/tag/v0.41.0-alpha.5
• The AutoFlow release workflow for v0.41.0-alpha.5 completed successfully: https://github.com/open-element/openelement/actions/runs/28499891702

The post-release package condition for #120 is now satisfied by the published 0.41.0-alpha.5 package line.