chore: remove unused sam3_bbox_mlp, sam3_params::seed, SAM3_VISUAL enum

[PABannier]

Removes code with no callers:

Symbol	Why it's dead
sam3_bbox_mlp (static)	never called
sam3_params::seed	never read (tc.seed in tests/test_metal_conv_transpose_2d.cpp is an unrelated local struct)
SAM3_MODEL_SAM3_VISUAL	never assigned, so sam3_get_model_type() never returns it. Visual-only SAM3 still reports SAM3_MODEL_SAM3, and sam3_is_visual_only() returns true. The other enum values keep their explicit numbers.

Kept: sam3_state_set_orig_dims. An earlier revision of this PR removed it, which was only safe on top of #20 (remove tests/). #20 is closed, so tests/test_sam2_pvs_compare.cpp still calls the function through an extern declaration and it has to stay.

Rebased onto main; no longer stacked on anything.

Verification: a Release build with -DSAM3_BUILD_TESTS=ON builds the library, all examples and all 43 test targets with no errors or new warnings.

Output is unchanged: a parity tool runs encode, PVS point + box, and a 3-frame visual track, then compares byte for byte against 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

git diff --shortstat main: 2 files changed, 15 deletions(-).

Part of an over-engineering cleanup series.

🤖 Generated with Claude Code

https://claude.ai/code/session_01M2qLmQ9ag49hT61V6i9qAq

[PABannier]

Rebase on main, and check the change is still useful.

[PABannier]

Rebased onto main (base retargeted from the closed cleanup/remove-test-api), and you were right to ask — one of the four removals is no longer valid.

Since #20 was closed, tests/ stays on main, and tests/test_sam2_pvs_compare.cpp:15 declares sam3_state_set_orig_dims via extern and calls it at line 49. I've kept that function; removing it would break the tests build.

The other three are still dead on current main:

Symbol	Check
sam3_bbox_mlp	git grep -w sam3_bbox_mlp → only its own definition
sam3_params::seed	only the field declaration; tc.seed in test_metal_conv_transpose_2d.cpp is a different local struct
SAM3_MODEL_SAM3_VISUAL	only the enumerator; never assigned, so sam3_get_model_type() never returns it

Now 15 deletions across 2 files, down from 20.

Verified: a Release build with -DSAM3_BUILD_TESTS=ON builds the library, all examples and all 43 test targets with no errors or new warnings. Output is unchanged too — encode, PVS point + box, and a 3-frame visual track match main byte for byte on EdgeTAM f16 and SAM2.1-tiny q8_0, CPU and Metal (PARITY OK ×4).