[REFACTOR] Rename shared symbolic analysis namespace to sym

[tqchen]

Rename the shared symbolic expression analysis and simplification namespace from arith to sym. Update C++ headers and sources, Python bindings, FFI registrations, build references, and documentation consistently.

Preserve historical serialized analysis objects through the existing JSON upgrader.

[tlopex]

Just curious, why we change arith to sym?

[tqchen]

Here sym means symbolic analysis. The module already covers symbolic expression simplification, bounds and sets, constraints, and iteration maps shared across the IRs, so the name makes that common role more explicit. arith is also reasonable; this is a naming clarification, with no change to functionality or dialect ownership.