* fix(local): match core's MMR tie-breaking
Local mode ordered MMR results differently from core whenever two
candidates tied exactly, on relevance or on MMR score. The MMR score
itself was already correct; the selection rules around it were not:
* the first point was seeded from `candidate_ids[0]`, i.e. whatever
`search` happened to return first, instead of the most relevant
candidate;
* `np.argmax` picked the *first* maximum, while core's `max_by_key`
returns the *last* one on ties;
* pending candidates were kept in an order-preserving list, while core
holds them in an `IndexSet` and drops the selected one with
`swap_remove`, which moves the last candidate into the freed slot and
therefore changes the order candidates are visited in - and so which
one wins a tie.
Reproduce the three rules in `_mmr`. The divergence was spotted on a
MAX_SIM multivector field with DOT, but it is specific to neither:
plain dense vectors and EUCLID diverge the same way once an exact tie
is constructed.
The added congruence tests keep relevance scores distinct on purpose:
core orders equally relevant candidates by search order, which is not
stable, so only ties in the MMR score can be asserted on.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
* refactor: move utils from collection, move tests
* tests: update comments
---------
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
