Tighten parser option handling

[codeforester]

Summary

• reject unknown std_run --options before attempting command execution
• keep std_run -- as the escape hatch for command names beginning with --
• allow arg_parse value options to consume literal -- as data
• document both parsing edge cases

Fixes #104

Validation

• bats lib/bash/std/tests/lib_std.bats
• bats lib/bash/arg/tests/lib_arg.bats
• ./tests/validate.sh
• git diff --check