Use array-api-compat for runtime namespace discovery

[jc-macdonald]

Closes #227.

Summary:

• centralize runtime namespace discovery in the public op_system.array_namespace helper backed by array-api-compat
• route scalar, vectorized, PyTree, history-body, propensity, and axis-kernel paths through the shared helper
• preserve scalar and vectorized autograd graphs by stacking native results and avoiding redundant conversion of existing arrays
• add an optional torch extra plus Torch-gated flat, PyTree, and axis-kernel autograd contracts
• keep the stricter static Array protocol unchanged pending coordinated protocol work in Flepimop2 #345, while documenting the broader runtime contract

Validation:

• 536 passed, 5 skipped with JAX installed
• Torch 2.14: 3 passed with UserWarning treated as error
• provider: 69 passed, 1 skipped
• strict mypy: core and provider passed
• Ruff format and lint passed repository-wide
• strict MkDocs build passed
• core and provider wheel build/Twine checks passed
• clean-wheel tests: core 509 passed, 14 skipped; provider 69 passed, 1 skipped