Two CLI-UX defects the Stage-1 R fieldtest surfaced (#131):
1. Tap discovery. `aura chart <name> --tap <bad>` rejected an unknown tap but
the error omitted the valid names. filter_to_tap now enumerates the run's
available taps in the message ("... (available: equity, exposure)"), so the
user can correct a typo instead of guessing -- refuse, but help.
2. Uniform --help. `aura chart --help` errored with "unexpected chart argument
'--help'" (exit 2) and `aura run --help` leaked usage to stderr via the
error path, while bare `aura --help` printed to stdout (exit 0). main() now
intercepts `--help`/`-h` anywhere in the invocation, prints usage to stdout,
and exits 0 -- uniform for every subcommand. The now-dead bare-flag match arm
is removed (the early intercept covers it).
RED-first: the existing chart_tap_nonexistent test gains an available-taps
assertion (was: message had no names), and a new per_subcommand_help test pins
`aura <sub> --help` -> stdout usage + exit 0 for run/chart/sweep/walkforward/
mc/graph/runs (was: chart --help exited 2). Unknown-subcommand stays exit 2.
Verified: clippy --all-targets -D warnings clean; full suite 514 passed / 0
failed (one new test); live `aura chart --help` prints usage to stdout, exit 0.
closes#131
Brummel
5c6de45b6e