fix: support tmpdir: false when the out dir is inside the project dir

[claude]

Requested by Samuel Attard · Slack thread

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Summarize your changes:

Before: packaging with tmpdir: false fails with ERR_FS_CP_EINVAL ("cannot copy ... to a subdirectory of self") whenever the out dir is inside the project dir — including the default ./out used by Electron Forge.

After: the same options package successfully, and the out dir is still excluded from the copied app.

How: with tmpdir: false the staging path is the final out path, so copyTemplate()'s single fs.cp(opts.dir, resources/app) has a destination inside its source. Node's cp rejects dest-inside-src up front, before the copy filter (which already ignores out dirs) ever runs. When the resolved destination is inside opts.dir, copyTemplate() now copies each top-level entry of opts.dir individually, applying the same userPathFilter to the top-level entries themselves — so ignored entries like the out dir are skipped. All other cases keep the existing single cp call.

Includes a regression test that packages with tmpdir: false and out inside dir, and asserts the out dir is not copied into resources/app.

Fixes #1679

Supersedes #1657 — this reimplements the per-entry copy approach from that PR (credit to @ianho, who is credited as commit co-author) on top of the current fs.promises.cp-based code.

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