Tighten std_run and arg_parse option handling

[codeforester]

Context

Claude's latest review found parser behavior that is technically functional but
surprising for users:

• std_run --typo ... treats the typo token as the command name and then fails
  as command-not-found.
• arg_parse rejects -- as the value for a value-taking option, even though
  the option is already known to require the next token as data.

Base is currently the only consumer, so we can tighten the behavior directly.

Scope

• Make std_run reject unrecognized --* option tokens with a clear error
  before command execution.
• Allow arg_parse value options to accept -- as a literal value.
• Add focused regression tests and update docs if needed.

Validation

• Focused BATS suites for touched libraries.
• Full ./tests/validate.sh.