fix(diff): pair colliding facts positionally instead of last-write-wins

[dejo1307]

factKey is not unique and no prop can make it so: Swift declares the same class twice under mutually exclusive #if/#else branches, overloaded methods share a name and symbol_kind, and a dependency fact's name already embeds its import target, so two imports of one target in one file are identical in name, props and relations. Such facts differ only by line, and line may not enter identity (diff.go:388, TestCompute_LineShiftIsNotAChange).

Keying them into a map[string]Fact silently dropped all but one. Two symptoms:

• Deleting a dropped fact produced no diff entry at all. 172 facts across three corpus repos were absent from the diff's map entirely.
• The on-disk baseline and the in-memory current iterate facts in different orders, so the surviving representative differed between the two sides and diff_snapshot reported a changed fact between byte-identical fact sets.

groupByKey now buckets facts into map[string][]Fact, sorts each bucket by intraGroupOrder (line, propsJSON, relationsJSON), and pairs the two sides positionally. Line is used only as an ordinal within a group that already shares an identity; it never enters factKey, so a lone fact that merely moves is still unchanged.

sortAll orders by factSortKey rather than factKey. Once colliding facts both survive, factKey no longer totally orders them, and the collections are built by ranging over a map -- sort.Slice would have left their order to Go's randomized map iteration, breaking the package's byte-identical-output promise.

The KindRoute/KindStorage discriminators are kept. They are no longer load-bearing for correctness but still pair a route's GET and DELETE facts by method rather than by position.

No cacheVersion bump: internal/diff emits no facts. Verified -- facts.jsonl hashed identically before and after.