fix: free model backend, buffer and context when the model is dropped

[PABannier]

Bug

sam3_load_model returns std::make_shared<sam3_model>(), and sam3_model has no destructor. Nothing in the library or examples calls sam3_free_model, so dropping the last shared_ptr leaked the weight buffer, the ggml context and the backend. The loader's early return nullptr paths (after the backend/buffer were created) leaked the same way.

Fix

Construct the shared_ptr with a deleter that calls sam3_free_model and then deletes the model. sam3_free_model nulls every pointer it frees, so a caller that already freed the model manually is unaffected.

Lifetimes are safe: every example declares model before state/tracker, so their buffers are released before the backend.

Verification

A throwaway program loads sam2.1_hiera_tiny_q8_0 plus a state 5 times, drops them each time, and prints resident memory:

iter	CPU before	CPU after	Metal before	Metal after
0	55.0 MB	12.7 MB	86.1 MB	44.3 MB
1	97.5	12.9	128.4	44.4
2	139.8	12.9	170.7	45.3
3	182.1	13.8	213.0	45.3
4	224.4	14.1	255.4	45.3

Output is also unchanged: a parity tool runs encode, PVS point + box, and a 3-frame visual track, then compares the output 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

A failing load (missing file) still returns nullptr cleanly.

🤖 Generated with Claude Code

https://claude.ai/code/session_01M2qLmQ9ag49hT61V6i9qAq