Wire the full-body tracking C++ examples into the rig launcher so one
command brings up the whole session instead of a hand-started runtime:

- rigs/full_body.yaml: first consumer-only rig — no producer plugin and
  no collection_id; the printer (validity gate) and the MCAP recorder
  read XR_BD_body_tracking joints directly from the runtime. No params:
  the recorder keeps its own defaults, and a fresh timestamped .mcap is
  written per Enter-rerun.
- full_body_printer: print '[body tracking inactive]' once per second
  while body tracking is unavailable (limp mode), sharing the literal
  record_full_body already uses, so the rig's validity-gate pane is
  never silently mute.
- rig_tests: load + footgun tests for the shipped rig, mirroring the
  se3_tracker pair.
- docs: rig launch pointers in body_tracking.rst and
  mcap_record_replay.rst, a limp-mode troubleshooting entry, and
  rig.rst now documents the consumer-only rig shape (collection_id
  rendezvous scoped to producer rigs).

Build green (full_body_printer, record_full_body), rig tests 30/30,
pre-commit and Sphinx docs clean.

Signed-off-by: Jiwen Cai <jiwenc@nvidia.com>

Follow-up to the generic FullBodyTracker refactor (NVIDIA#781), which removed
deviceio_trackers/full_body_tracker_pico.hpp and renamed the Pico types:
FullBodyTrackerPico -> FullBodyTracker, FullBodyPosePicoT -> FullBodyPoseT,
BodyJointPico_* -> BodyJoint_*. full_body_printer and record_full_body now
use the new header and names; no behavior change (the session still
selects the default vendor).

Both targets build, rig tests 30/30, clang-format-14 and pre-commit clean.

Signed-off-by: Jiwen Cai <jiwenc@nvidia.com>

Running the full_body rig dropped full_body_<timestamp>.mcap into the
repo root (the pane cwd), showing up as untracked noise in git status.

- record_full_body: the output argument may now be a directory (existing
  or with a trailing slash) — it is created if needed and each run writes
  a fresh timestamped file inside it, so reruns still never clobber.
- rigs/full_body.yaml: point the recorder at
  examples/mcap_record_replay/recordings/ — already gitignored and the
  default search location of the replay workflow, so a bare replay
  invocation picks up rig takes automatically.
- body_tracking.rst: document where takes land.

Verified all three output modes (dir arg creates the directory and
timestamps inside; explicit .mcap path unchanged; bare default
unchanged); rig tests 30/30; clang-format-14 and pre-commit clean.

Signed-off-by: Jiwen Cai <jiwenc@nvidia.com>