ema: relocate to correct source root

The file was accidentally placed at fairchem/src/fairchem/core/common/ema.py
(a stray fairchem/ prefix from a checkout confusion), which is outside the
paths ruff.toml globs — so its per-file-ignores never applied and, when
CI enumerated the file explicitly, it was treated as if outside the
project include, tripping several rules.

Move to src/fairchem/core/common/ema.py where the rest of the module
lives. Verified: `ruff check` / `ruff format --check` both pass under
the project's ruff.toml at the new path.

Author: bkmi user