[alpha.5] T2.1 Define BuildPlan / BuildArtifacts shapes in protocol

[SisyphusZheng]

Parent\n\n#130\n\n## What to build\n\nAdd BuildPlan and BuildArtifacts type contracts to @openelement/protocol so the rest of the alpha.5 build refactor has a stable interface.\n\n## Acceptance criteria\n\n- [ ] BuildPlan captures route inputs, island inputs, output options, i18n/content options, and package-island options.\n- [ ] BuildArtifacts captures generated HTML, manifests, client assets, warnings, and errors.\n- [ ] Unit tests assert the shapes are complete and serializable.\n- [ ] Types are re-exported from @openelement/ssg and consumed by @openelement/adapter-vite without duplication.\n\n## Blocked by\n\n- #129

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Closing as completed or superseded by the alpha.5 train.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SPA mode + client router + desktop proof) was merged and its Alpha.5 Issue Coverage table marks the alpha.5 items done/superseded and lists them for closure: feat(alpha.5): SPA mode + client router + desktop proof #121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