feat: add Ascend950 direct SDMA backend

[Kur0x]

Summary

• add a runtime-selected Ascend950/A5 SDMA backend using CANN's built-in ShmemSdmaStarsQuery, 48 STARS channels, Host RTSQ mappings, and direct two-SQE AIV submission
• preserve the existing A2/A3 PTO path and best-effort communicator behavior, including retryable resource cleanup and per-communicator failure isolation
• migrate the supported driver floor to 25.1.rc1, extend the SDMA demo/tests and CI with an Ascend950 compile-only gate, and update both the draw.io source and exported SVG

Validation

• python3 -m unittest discover -s tests/ci -p 'test_*.py': 141 passed, 1 skipped
• ruby tests/ci/test_workflows.rb: passed
• bash tests/ci/test_common_env_override.sh: passed
• C++14 compile and execution: SDMA metadata, A5 query validation, communicator wiring, and source guard tests passed
• bash syntax checks, git diff --check, and draw.io/SVG XML validation passed
• exported architecture SVG visually inspected; no clipping or layout regressions
• independent code review completed after fixes; final focused review reported no findings

Remaining Validation

• a full CANN 9.1 Host build and dav-3510 compile must run in CI; the local macOS workspace has no CANN installation
• the documented Ascend950 data-plane matrix was not rerun in this task because the validation host was unavailable
• the approved plan's fully injectable Host runtime failure matrix is not yet implemented; this PR remains draft until that gap and CI results are resolved

Base

• targets main directly; the branch contains one feature commit on top of origin/main

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