[AI] Fix object mask concurrency and shape-lifetime bugs

[andriiryzhkov]

Two open PRs by @sjuxax have been awaiting a response from the author for a while. I don't consider everything in those PRs meaningful for merging, but a couple of fixes are genuinely worth having. This PR ports only those.

The originals:

• #20774 – Fix object mask ASan races and scratch overflows

  • Ported: the _run_decoder thread-join fix. Real concurrency bug.
  • Skipped: per-iteration malloc/free replacements of dt_get_perthread() in box_filters.cc and guided_filter.c. These patch symptoms rather than the root cause (likely a pool-sizing issue in dt_alloc_perthread_float), and the hot-path allocator churn would regress performance for every user, not just those running ASan.

• #20775 – Fix SAM2 segmentation ASan crashes

  • Ported: the decoder shape-array lifetime fix. Genuine UB per C standard and zero runtime cost.
  • Skipped: DT_AI_OPT_DISABLED for the SAM2.1 encoder. This disables ORT graph optimization for all SAM2 users to work around an ORT 1.24.x UAF that may be ASan-only, at a measurable inference-performance cost. Worth revisiting if confirmed to crash in release builds.

What's in this PR

1. Wait for encode thread before decoding. ENCODE_READY is published before the background warmup finishes, so a fast user click could race the warmup on the shared dt_seg_context_t. _run_decoder() now joins the thread first – immediate no-op if warmup already completed, brief pause otherwise. Related comment at the ready-signal site was updated to reflect the new behavior.

2. Keep decoder output shape arrays alive through dt_ai_run. In dt_seg_warmup_decoder and dt_seg_compute_mask, iou_shape / lr_shape / low_res_shape were declared inside an inner if(is_sam) block while pointers to them were stored in an outputs[] array from the outer scope. Per C, their lifetime ended at the closing } of the inner block – the dt_ai_run() call afterward read through dangling pointers. Works in release today because compilers rarely recycle stack slots between sibling scopes, but caught by ASan and a latent time bomb for future compiler versions. Moved to the outer scope; zero runtime cost.

[TurboGit]

Thanks!