refactor: deduplicate run-level option propagation and document the resolution model

[sheremet-va]

Description

coverage, attachmentsDir, and mergeReportsLabel were copied from the root config to projects in three different places: resolveTestConfig (coverage), the vitest:project plugin's configResolved hook (coverage again, plus attachmentsDir), and a one-off mergeReportsLabel assignment in resolveSingleProjectEntry. The copies now live in resolveTestConfig only: coverage keeps its existing early assignment (it is shared by reference so projects append to the shared coverage.exclude), and attachmentsDir/mergeReportsLabel are assigned at the end of the resolution. The configResolved copies were redundant: nothing reads the Vite-level test values between the project's Vite resolution and resolveTestConfig.

The advanced guide gains a "Project Configuration Resolution" section documenting how startVitest/createVitest arguments interact with projects: what extends does and does not carry (the config file only, not viteOverrides or CLI options), which option groups reach every project regardless of extends, and why plugins only transfer through a config file.

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