feat: add shared TTY progress bars for ZED offline tools

Extract the tqdm-like stderr progress bar into a shared helper and reuse it across zed_svo_to_mp4, zed_svo_grid_to_mp4, and zed_svo_to_mcap.

For zed_svo_to_mcap, single-source exports now report exact frame totals and bundled multi-camera exports report exact synced-group totals on TTY. When bundled mode runs without --end-frame, the tool performs a counting pass first so the progress total remains exact instead of estimated.

Also document the bundled MCAP progress behavior in the README and record the current third_party dependency state in third_party/README. That note now makes it explicit that CLI11 and proxy are the active submodules, while tomlplusplus and mcap are vendored source drops, and adds a TODO to revisit converting mcap into a submodule later.

Verified with the Debug build during implementation, including single-camera and bundled zed_svo_to_mcap runs that rendered clean progress output to a TTY.

README.md

@@ -264,6 +264,7 @@ uv run python scripts/zed_batch_svo_to_mcap.py \
 The batch MCAP wrapper writes `<segment>/<segment>.mcap` by default, skips existing outputs unless told otherwise, and returns a nonzero exit code if any segment fails.
 The repo includes a minimal pose config at `config/zed_pose_config.toml` so MCAP conversion does not depend on a separate `cv-mmap` checkout.
 In bundled multi-camera mode, `--end-frame` means the last emitted synced frame-group index from the common start timestamp.
+When stderr is attached to a TTY, `zed_svo_to_mcap` shows a tqdm-like progress bar. In bundled mode without `--end-frame`, it first counts synced groups to make that progress total exact.
 
 Use `--probe-existing` to validate existing MCAPs before skipping them. Invalid outputs are treated as missing and requeued: