Rename package synapt_eval → synapt.eval (PEP 420 namespace)

[laynepenney]

The eval package currently builds as a flat top-level package synapt_eval, which breaks the shared synapt.* PEP 420 namespace it should co-inhabit with recall (synapt.recall), extract (synapt.extract), premium (synapt.premium), and runner (synapt.runner).

Current state (verified 2026-07-21):

• dist name synapt-eval — correct (hyphen)
• [tool.hatch.build.targets.wheel] packages = ["src/synapt_eval"] — WRONG
• layout eval/src/synapt_eval/ → imports as synapt_eval, so import synapt.eval fails

Fix:

• move src/synapt_eval/ → src/synapt/eval/
• set hatchling to build the synapt/ namespace dir (e.g. packages = ["src/synapt"]), no __init__.py at src/synapt/ (namespace package)
• keep dist name synapt-eval (hyphen)

Why now: grep found zero .py files importing the package yet — no call sites to update. Cheap now, expensive once code accrues. (Runner is already correct: synapt/runner/ → synapt.runner, no change needed.)

Three spellings must stay consistent: PyPI synapt-eval / import synapt.eval / dir src/synapt/eval/.

Premium boundary: OSS (eval framework is public synapt-eval).