refactor: drop sam3_visual_track_params in favour of sam3_video_params

[PABannier]

sam3_visual_track_params duplicated 4 fields of sam3_video_params (assoc_iou_threshold, max_keep_alive, recondition_every, fill_hole_area), with the same defaults. sam3_create_visual_tracker immediately copied them field by field into a sam3_video_params, because that's what the tracker stores. Now it takes sam3_video_params directly and clears text_prompt, so a visual tracker still never runs PCS detection.

Public API change: sam3_create_visual_tracker(model, sam3_video_params). The README's sam3_create_visual_tracker(*model, {}) compiles unchanged. In-repo callers are updated:

• examples/benchmark.cpp: changes the local's type.
• examples/main_video.cpp: drops the mirror struct and the 4-line copy, and passes app.track_params directly.

Behaviour: the tracker used to ignore the caller's score_threshold, nms_threshold and hotstart_delay and apply the defaults (0.5 / 0.1 / 15). It now honours them. Every current caller passes the defaults for those fields (main_video never edits them), so nothing observable changes.

Stacked on #20 because tests/ on main still uses the old struct. Merge #20 first; GitHub will retarget this PR to main.

Verification: a parity tool calls sam3_create_visual_tracker(*model, {}), then compares add_instance plus a 3-frame propagation (and PVS) byte for byte with origin/main:

PARITY OK   edgetam_f16 gpu0
PARITY OK   edgetam_f16 gpu1
PARITY OK   sam2.1_hiera_tiny_q8_0 gpu0
PARITY OK   sam2.1_hiera_tiny_q8_0 gpu1

The full Release build, including sam3_benchmark and sam3_video, has no errors or new warnings.

Follow-up worth a look: recondition_every is never read anywhere in sam3.cpp; it's dead config in sam3_video_params too.

git diff --shortstat: 4 files changed, 8 insertions(+), 25 deletions(-).

Part of an over-engineering cleanup series.

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https://claude.ai/code/session_01M2qLmQ9ag49hT61V6i9qAq