feat: add eqOrIn and neOrNin query value utils - #49

Summary

Adds two utils for a pattern that recurs whenever you build a query from a
list of values: collapse to the narrower operator when only one value remains.

eqOrIn([1, 2, 3]) // => { $in: [1, 2, 3] }
eqOrIn([1]) // => 1

neOrNin([1, 2, 3]) // => { $nin: [1, 2, 3] }
neOrNin([1]) // => { $ne: 1 }

onDelete already hand-rolled exactly this branch and now uses eqOrIn
instead.

An empty list yields { $in: [] } / { $nin: [] } — semantically correct
(matches nothing / excludes nothing), but documented, since some adapters
dislike an empty IN ().

Other Information

Deduplication. Values are deduplicated before the length check, so
[1, 1] collapses to the single-value form too. This is behaviour-preserving:
a deduplicated $in matches the same set of records.

The dedupe lives in a new shared src/common/dedupe-values.ts (in-source
tests) used by both utils. It compares primitives with SameValueZero and
non-primitives deep-equal via dequal — the dependency dedupeBranches
already uses for the same job on $and/$or branches. Deep-equal matters for
the most common real case: result.map((x) => x[keyHere]) over a mongo service
yields distinct ObjectId references with equal values, which a plain Set
would not collapse. Same for Date.

Primitives never reach the deep comparison, so a large list of scalar ids stays
a single Set pass and only the non-primitive elements are compared pairwise —
relevant because $in arrays can hold thousands of ids, unlike the handful of
branches dedupeBranches sees.

One documented trade-off: dequal ignores key order, so two plain objects with
the same entries in a different order count as one. MongoDB matches document
values in $in field-order-sensitively and would treat them as distinct. Plain
documents as $in values are vanishingly rare in Feathers, and
dedupeBranches already makes the same trade-off, so this seemed the right
default — happy to restrict deep-equal to non-plain objects if preferred.

Docs pages are auto-discovered from the sibling .md files; test/index.test.ts
export lists updated. pnpm test passes: 0 lint errors, no type errors, both
utils and the helper at full coverage.

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