Add :fix_window_per_key algorithm for ETS and Atomic backends

[epinault]

Closes #181.

Summary

Adds :fix_window_per_key to both the ETS and Atomic backends. It's a fixed-window variant whose window is anchored
to each key's first hit instead of a globally-aligned wall-clock epoch (multiples of scale since Unix epoch,
like :fix_window uses today).

Same one-entry-per-key memory profile as :fix_window. The 2× boundary burst is still theoretically possible per
key, but boundaries are no longer globally synchronized — they can't be timed deterministically by an attacker, and
a key has to wait a full scale between burst opportunities. Same semantics as the common Redis INCR + EXPIRE NX
pattern.

Naming chosen deliberately over :rolling_window — the algorithm is mechanically a fixed window, just with a
per-key epoch. The docstring is explicit that the 2× burst is still possible, so users reaching for this won't
expect :sliding_window-level precision.

What's included

• Hammer.ETS.FixWindowPerKey and Hammer.Atomic.FixWindowPerKey
• Dispatch wiring in Hammer.ETS and Hammer.Atomic (incl. before_clean algorithm name)
• Atomic backend reuses the existing clean_fix_window path (identical row layout)
• Test files mirroring the existing fix_window tests, plus distinguishing tests for per-key window anchoring,
  expiry behavior, and the atomic race condition
• before_clean callback tests in both clean_test.exs files
• README algorithm table, comparison section, and selection guide updated
• CHANGELOG ## Unreleased entry

Test plan

• mix test — 129/129 passing
• mix credo --strict — clean
• mix format --check-formatted — clean