[alpha.5] T2.3 - Migrate SSG tests to the build() interface

[SisyphusZheng]

Parent

#130

What to build

Refactor existing @openelement/ssg tests so they exercise behavior through build() instead of calling scattered internal helpers.

Acceptance criteria

• Internal helper tests that can be expressed through build() are migrated.
• No regression in coverage.
• Full test gates pass.

Blocked by

• [alpha.5] T2 - Deepen SSG around BuildPlan and BuildArtifacts #130-T2.2

[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Closing as completed or superseded by the alpha.5 train.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SPA mode + client router + desktop proof) was merged and its Alpha.5 Issue Coverage table marks the alpha.5 items done/superseded and lists them for closure: feat(alpha.5): SPA mode + client router + desktop proof #121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.