Fixes single import types defintion requirement for docs

[clairebaire]

No description provided.

[Copilot]

Pull request overview

Updates the packaging build script to ensure the generated root type definitions reference all API-area modules, addressing docs tooling that only reads the root types entry from the package exports map.

Changes:

• Discovers “API area” subdirectories under src/ (directories containing index.ts) once and reuses the list for multiple packaging steps.
• Appends type-only, namespace-style re-exports for each API area into the generated root index.d.ts so docs tooling can “see” all areas through a single types entry.
• Refactors package.json#exports generation to use the shared API-area discovery list.

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[craftytrilobite]

Some of the comments are difficult to understand/verify- e.g. "// Skips Common because src/index.ts already wildcard-re-exports it at the root for backward compat," but I don't see anything terribly dangerous here- approved.