Document mir_history offset; fix waypoint assertion

[neuromechanist]

Closes #161, and repairs a test my own #241 broke.

#161 was mostly already done

Both of its testable claims already have tests in test_ng_convergence.py — test_mir_history_survives_keep_best_restore and test_mir_history_empty_after_save_load. Only the third, informational item was outstanding.

Verified before documenting it: ll comes from the E-step accumulator at the top of the loop, _update_parameters runs, and the MIR waypoint is computed after. So mir_history_[i] reflects the parameters after iteration i's update while ll_history[i] is the likelihood of the parameters before it. Zipping them by index compares different parameter states. Now stated on both the backend attribute and the AMICA.mir_history_ docstring.

A regression from #241

Running the whole file rather than a subset surfaced test_mir_history_survives_keep_best_restore failing on dev:

assert 50 == (55 - 1)

I bisected it: the test passes at 257d20e (the commit before #241 merged) and fails at c80fb33 (the merge). #241 caused it. That PR's gm-ordering change shifts multi-model trajectories by ~1e-6, which moved the min_dll stop to iteration 55; MIR waypoints land on multiples of mir_step=5, so the last one is at 50, not 54.

The assertion last_it == len(ll_history) - 1 was fragile from the start — it only held while the stop happened to coincide with a waypoint, and any trajectory perturbation could break it. It now asserts what the test is actually about: the last waypoint is the highest multiple of mir_step at or below the final iteration, and is within one interval of the end, so a restore that truncated the trajectory would still fail it.

Why CI did not catch it

The test is not slow-marked, so CI runs it, and CI was green on #241's branch. I have not established why the branch and the squashed merge behave differently here; the practical lesson is that I ran a subset locally on #241 and would not have seen this either way. Flagging rather than leaving it implicit.