Feature gap: several blackbox_decode CLI output/unit flags have no bbl_parser equivalent

[nerdCopter]

AI Generated issue-ticket

Summary

A CLI-flag parity sweep of betaflight/blackbox-tools' blackbox_decode.c
against bbl_parser's current src/main.rs CLI (--debug, --output-dir,
--gpx/--gps, --event, --force-export, --version) turned up several
blackbox_decode flags with no bbl_parser equivalent, none tied to IMU/current
simulation (tracked separately in #49 and #50):

• --index <num> — decode/export a single selected log from a multi-log file.
  bbl_parser always decodes and exports every log in the file (.01.csv,
  .02.csv, etc.) with no way to select just one.
• --limits — print the min/max range of each field without doing a full
  export. Not implemented.
• --stdout — write CSV output to stdout instead of a file. Not implemented;
  bbl_parser always writes to disk.
• --unit-amperage, --unit-vbat — bbl_parser hardcodes conversion to A/V
  (src/conversion.rs) with no raw/mA/mV override.
• --unit-flags — no raw-vs-decoded-flags toggle for state/flight-mode flags.
• --unit-frame-time, --unit-height, --unit-rotation, --unit-acceleration,
  --unit-gps-speed — no user-selectable unit conversion; GOALS.md already
  lists "time scaling, altitude conversion, speed units, rotation rates,
  acceleration scaling" as missing under "Unit conversions expansion."
• --alt-offset — altitude offset override. Not implemented.
• --merge-gps — merge GPS data into the main CSV instead of a separate GPX
  file. GOALS.md already lists this as "not started" ("GPS data merge").
• --raw — export unprocessed/undecoded field deltas instead of applying
  predictors. GOALS.md already lists this as "not started" ("Raw mode
  output").

Verified --save-headers is not a gap — bbl_parser always writes a
.headers.csv file (src/export.rs), matching GOALS.md's claim that this is
complete.

Context

Found while investigating #67/#68 upstream in blackbox-tools and doing a
follow-up survey of full CLI parity, per user request. Grouping these together
since they're all independent, self-contained CLI flags rather than a
substantial feature like IMU or current-meter simulation.

Open question

None of these are a current priority. Filing for tracking only — not a
commitment to implement any of them.