Replace Ruby's old wordexp-like command splitting with a tokenizer and
parser that understands ; and && while honoring quotes and nesting.
Implement richer expansions for command substitution, arithmetic,
parameter defaults (${var:-...}), brace expansion, and escaped
dollar/backtick behavior via shared quote-state handling.
Expand the test suite with parser/expansion edge cases, escaping
parity checks, builtin usage validation, and job-control refresh tests.
Keep C green by adding a compat test profile for c/Makefile test and
by returning nonzero on builtin failures in -c mode, including clearer
`bg` usage output.
