fix(interpreter): populate BASH_SOURCE[0] for PATH-resolved scripts

[chaliy]

Summary

• When a script is executed via PATH lookup (by name, not by path), BASH_SOURCE[0] was empty because only the bare command name was passed to execute_script_content() instead of the resolved path
• Fixed try_execute_script_via_path_search() to pass the fully resolved candidate path
• Added 2 new tests: direct BASH_SOURCE check and source-guard pattern via PATH lookup

What changed

In try_execute_script_via_path_search(), the code already resolves the full path (candidate = PathBuf::from(dir).join(name)) but was discarding it when calling execute_script_content(name, ...). Now passes &candidate.to_string_lossy() instead.

Test plan

• bash_source_set_via_path_lookup — verifies BASH_SOURCE[0] equals /scripts/test.sh when run by name via PATH
• bash_source_guard_via_path_lookup — verifies source-guard pattern (BASH_SOURCE[0] == $0) works correctly via PATH
• All existing bash_source tests still pass (6/6)
• Full test suite green (2700+ tests)
• cargo fmt --check clean
• cargo clippy -- -D warnings clean

Closes #1085