Refactor SSA compiler naming

[voltjia]

Summary

• Replace legacy-style IR names with namespace-scoped kernel and SSA IR names.
• Move SSA frontend lowering, compiler passes, backend registry, and SSA emitter code into clearer packages.
• Drop the legacy ProgramIR compatibility path and update tests to cover the SSA-first API surface.

Validation

Remote validation was run on ssh nvidia from ~/ninetoothed.

~/.venv/bin/ruff check --fix
Found 11 errors (11 fixed, 0 remaining).

~/.venv/bin/ruff format
72 files left unchanged

~/.venv/bin/ruff check
All checks passed!

~/.venv/bin/python scripts/check_contributing_style.py
passed

pytest output:

PATH=$HOME/.venv/bin:$PATH ~/.venv/bin/pytest tests/test_backend_registry.py tests/test_kernel_ir.py tests/test_lowering_inference.py tests/test_ssa_application_lowering.py tests/test_ssa_first_backend_lowering.py tests/test_ssa_pass_pipeline.py
68 passed, 1 skipped in 40.52s