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

[SisyphusZheng]

Parent\n\n#130\n\n## What to build\n\nMove existing @openelement/ssg tests away from scattered internal call sites and toward the public build(plan) interface.\n\n## Acceptance criteria\n\n- [ ] Existing ssg tests exercise behavior through build() where possible.\n- [ ] Tests that must remain internal are marked with a ponytail note explaining why.\n- [ ] No drop in test coverage.\n- [ ] Full test gate passes.\n\n## Blocked by\n\n- #164

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Closing as completed or superseded by the alpha.5 train.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SPA mode + client router + desktop proof) was merged and its Alpha.5 Issue Coverage table marks the alpha.5 items done/superseded and lists them for closure: feat(alpha.5): SPA mode + client router + desktop proof #121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