docs(pack): document exe peer dependencies

[leno23]

Summary

• Document that vp pack executable builds load @tsdown/exe at build time.
• Show that @tsdown/exe must be installed with a matching top-level tsdown package because Vite+'s bundled tsdown is not resolvable as the tsdown package peer.
• Call out that installing only @tsdown/exe can surface as Failed to import module "@tsdown/exe" when the underlying unresolved import is tsdown/internal.

Refs #1586

Test Plan

• npm view @tsdown/exe@0.22.0 peerDependencies --json -> { "tsdown": "0.22.0" }
• Source proof: origin/main:docs/guide/pack.md did not contain the actionable @tsdown/exe@0.22.0 tsdown@0.22.0 note or tsdown/internal; patched docs contain both plus the vp env current version guidance.
• git diff --no-index --check /tmp/viteplus-pack-md-original.md /tmp/viteplus-pack-md-patched.md -> no whitespace errors
• pnpm --dir docs build -> build complete in 87.23s

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