Feature gap: no IMU attitude/heading simulation (no --simulate-imu equivalent)

[nerdCopter]

AI Generated issue-ticket

Summary

bbl_parser has no equivalent of blackbox-tools' --simulate-imu feature. There is no
complementary-filter attitude/heading estimation anywhere in the codebase — bbl_parser
only decodes BBL frames and exports the recorded fields (CSV/GPX/event), it does not
derive roll/pitch/heading from gyro/accel/mag samples.

Confirmed via full grep of src/ for imu, heading, attitude, EstM, EstG,
EstN, complementary, atan2, simulate — zero matches on current master (283c05e).

Context

This was noticed while investigating a heading-computation bug in blackbox-tools:

• Why heading is different that in other tools? Roll and Pitch match perfectly. betaflight/blackbox-tools#67
• fix: remove IMU heading warm-up transient by zero-seeding EstM betaflight/blackbox-tools#68

That bug (an arbitrary-seed warm-up transient in the magnetometer heading reference
vector) does not apply here, since bbl_parser doesn't implement the feature that bug
lives in.

Open question

Is --simulate-imu (attitude/heading estimation from gyro/accel/mag) in scope for
bbl_parser, or should it stay a pure decode/export tool? This is a scope discussion,
not a bug report and not a commitment to implement — no prior task or issue on this
topic existed before the recent blackbox-tools issue/PR prompted this investigation,
and there's no known personal use case for it yet.

[nerdCopter]

AI Generated comment

Also grouping in a related, smaller gap found during a broader blackbox_decode CLI parity sweep: --declination <val> / --declination-dec <val> (magnetic declination adjustment for magnetometer-based heading) have no bbl_parser equivalent either. These flags only have meaning once magnetometer-based heading computation exists, so they belong with this issue rather than a separate one.