fix: apply addon preparsers on initial load

[NgoQuocViet2001]

Summary

Addon setup/preparser.ts hooks were unavailable during Slidev's cold-start/static-build parse because resolveOptions() returned data loaded with only the project root.

This change:

• keeps the first parse as a bootstrap step for discovering the selected theme and addons
• reloads the deck with the complete theme/addon/project root set
• resolves the returned config from the final preparsed headmatter
• pins theme and addons to the graph that was actually loaded, while preserving final non-discovery fields and applying CLI overrides last

The regression makes bootstrap and final headmatter intentionally diverge, verifies the full-root second load, and catches stale-config or unloaded-root contradictions.

Closes #2646

Validation

• Focused regression: 1/1 passed
• Slidev Node tests: 61/61 passed
• Full test suite: 218/218 passed
• @slidev/cli build passed
• ESLint on the two changed files and git diff --check passed
• Real addon cold-start spot-check passed

GitHub Actions completed with 16 successful and 2 neutral checks. The only failures were typecheck and cypress; the same two jobs also fail on upstream main at base SHA 905b44ba (base run), while the PR's Windows/macOS/Linux tests and all smoke matrices passed.

Locally, pnpm verify likewise reaches unchanged Cypress/VitePress/duplicate dependency type errors and CRLF lint errors in unchanged Markdown files. The production build, complete test suite, and changed-file lint all pass.

AI assistance

Codex (GPT-5, autonomous) analyzed the issue, implemented and iteratively reviewed the change, and ran the validations above on behalf of @NgoQuocViet2001. The operator requested the contribution but did not manually author or line-by-line review the patch.

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@slidev/client

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commit: 643c23a