fix: throw a descriptive error when the out dir would exclude the app from packaging

[claude]

Requested by Samuel Attard · Slack thread

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Summarize your changes:

Before: if the out directory resolved to a location inside the app source — e.g. main: "dist/code.js" in package.json with --out ./dist, or --out pointing at the app directory itself — the out directory was silently excluded from the copy (out dirs are always excluded from packaging), and the user got a misleading error blaming their ignore option: The main entry point to your app was not found. Make sure "..." exists and does not get ignored by your ignore option (or the Application manifest was not found variant when out equals dir).

After: packaging fails with a targeted error that explains the actual problem, e.g.:

The out directory (/path/to/app/dist) is inside your app directory and contains your app's main entry point (/path/to/app/dist/code.js). The out directory is automatically excluded from packaging; choose an out directory outside of your app directory

and, when out is the app directory itself:

The out directory (/path/to/app) is the same as your app directory. The out directory is automatically excluded from packaging, so nothing would be packaged; choose an out directory outside of your app directory

How: App.buildApp (src/platform.ts) now passes the resolved ignored out dirs (from generateIgnoredOutDirs in src/copy-filter.ts) into validateElectronApp (src/common.ts). When validation fails and the missing manifest/main entry point falls inside an ignored out dir, the new targeted error is thrown instead of the generic one; behavior is unchanged for every configuration that packages successfully today. Covered by new unit tests in test/common.spec.ts (plus a new fixture) and end-to-end tests in test/packager.spec.ts for both the out-dir-contains-main-entry-point and out-equals-dir cases.

Fixes #1685