Add an input configuration option for declaring application entry points once and using them consistently in development and builds.
The option must be available in shared environment configuration and accept a path, an array of paths, or an alias-to-path map. Resolve every configured path relative to the project root. A root-level value supplies the client environment; it must not implicitly leak into other environments, which can declare their own independent input values.
Use input as the default entry source for normal and multi-page builds, environment-specific builds such as SSR, library builds when build.lib.entry is omitted, and development dependency discovery. An explicit build.rolldownOptions.input must continue to take precedence for builds.
Ensure root-level inputs compose sensibly through mergeConfig instead of dropping either configuration. In particular, when positional string/array entries are combined with an alias map, preserve them under aliases derived from their file basenames and retain the map's aliases.