feat(api): add semantic Task C ABI and C++ SDK

[yifeif-nv]

Background

Prepare the shared Task SDK so subsequent family migrations can stay inside
families/<name>/**, without editing Core, applications, website registration
or CI for an already-supported capability. This remains a draft while the
remaining migration rehearsals and performance-reference gaps are resolved.

Exit Criteria

• All existing family capabilities have the required shared contracts and
  application paths, with family-only migration evidence.
• C ABI ownership, configuration, lifecycle and installation checks pass;
  existing workloads, BYOK, examples and benchmark coverage remain available.
• Applicable CI passes on the final PR head. Production family migrations and
  the actual merge are not part of this update.

Implementation

• Families return typed bind<Interface>(*this, fields) records. Core snapshots
  the adjusted interface pointers and configuration metadata, then calls the
  internal interfaces directly. No additional adapter framework or family registry.
• Shared configuration checks reject unknown/duplicate names and wrong types
  before execution. Families retain defaults, ranges and input-dependent policy.
  Missing values remain distinct from explicit zero, false or empty values.
• Complete additional detection/structure contracts and shared CLI, benchmark,
  report and website paths. Preserve batch/session/callback ownership and error
  precedence. Unknown embedding-space identity is explicitly empty, not fabricated.
• Correct Conan's SDK install directory so the wheel's C library and native
  runtime are colocated. Keep the existing full-family packaging checks.
• Four existing checkpoint-policy tests move into their owning family test
  folders, retaining assertions and automatic CPU collection. No production
  builder/runtime migration is included.

Change categories

• Model or runtime behavior
• Public API
• ABI
• Bundle or artifact format
• Dependencies
• Documentation only
• CI or developer tooling

SDK/wheel payloads change; bundle format 1 does not. Pandas remains optional.
No runner configuration, new exclusions, precision thresholds or passing
criteria are changed by this PR.

Validation

Commands and Results

Local evidence, not a claim that the updated remote head has passed CI:

• python3 -m tools.community_ci unit: the current 126-family source snapshot passed
  946 Python tests (1 optional-dependency skip), built all native targets, and
  passed 198 CTests (6 existing no-GPU skips), including the latest embedding
  metadata change.
• python3 -m tools.community_ci source-quality --base 714f1fc0d567213a7b79a488e9dcfdf405279c6a:
  passed legal, inventory, complexity and formatting checks plus 178 tests.
• PYTHONPATH=core/builder:apps/benchmark:. python3 -m pytest apps/benchmark/trtmc_benchmark/tests/test_perf_matrix.py apps/benchmark/trtmc_benchmark/tests/test_timing_contracts.py -q:
  103 passed. Existing mainline release entries/exclusions are retained.
• ctest --test-dir BUILD_DIR --output-on-failure -R '^test_features_(c|cpp)_api$':
  2 passed, including explicit unknown identity and result lifetime tests.
• Retained pre-binding C/C++ client binaries run against the new library
  without recompilation. The existing single-device BYOK round trip passed.
• npm --prefix website run build: passed, including 34 diagram checks.
• Full wheel/archive and installed-SDK checks passed on the earlier 124-family
  snapshot; the newly added mainline families still need the final package refresh.

Hardware, Environment, and Revisions

Current development base: public main
714f1fc0d567213a7b79a488e9dcfdf405279c6a (126 families, 276 manifests).
Tested source is the content committed as
aa909bfd4207962dd2376bf29dd81abc63ff174d.
Local CPU validation uses the repository's existing isolated CPU image,
Linux/aarch64, GCC 13.3, CUDA 13.3 development tools and TensorRT 11.1.
Protocol fixtures are separate from trained-model numerical qualification.

Not Run / Remaining Gaps

• Complete the remaining family-only rehearsals and matched performance
  references; no component timing or recorded output is presented as full inference.
• Refresh the complete wheel and applicable checks for the final revision.
• Final-head public CI, protected premerge and Nightly are not claimed here.
• No multi-device execution. Earlier single-device model rehearsals do not
  establish qualification of every model or the final revision.

Contributor Self-Review

• I have completed a self-review of this change.

Reviewed contracts, lifecycle/error handling, ownership and publication
boundaries, including independent code and confidentiality reviews.

Notes For Future Readers

Start with website/docs/api/cpp-api.md and the family migration checklist in
website/docs/extend/add-model-family.md, then the public headers and tests.
The SDK remains experimental until its first stable release. Core/backend/family
DSOs upgrade together; applications compile the convenience C++ header against
the public C ABI. Existing family execution paths remain during migration;
failed SDK calls never retry an older path.

Risk level

• Low
• Medium
• High

This is a broad ABI/application change. CPU protocol coverage and isolated
rehearsals are necessary but do not replace real-model qualification.

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