#568 Fix piped installer under nounset

What

• Make the installer source guard tolerate an unset BASH_SOURCE[0] when Bash reads the script from standard input.
• Add a regression test that pipes the real installer into Bash without downloading or installing anything.

Why

The documented curl ... | bash installation path runs with BASH_SOURCE[0] unset. The source guard added in #558 dereferences that value while set -u is active, so the installer exits before main runs. Falling back to $0 preserves direct and piped execution while still preventing main from running when the script is sourced.

Testing

• bats e2e/installer.bats
• bin/ci

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Summary by cubic

Fix the installer so piping to bash works under set -u. The source guard now tolerates an unset BASH_SOURCE[0], so curl ... | bash runs main as expected.

• Bug Fixes
  • Update guard to [[ "${BASH_SOURCE[0]:-$0}" == "$0" ]] to handle stdin execution while still preventing execution when sourced.
  • Add e2e test that pipes the real installer to bash, runs with the current $BASH, and asserts it fails on missing curl (not on an unbound variable).