[koss-null]

Mostly bug fixes that was found during extensive usage of the library and adding AI tools to the development pipeline.
The full list:
Correctness
Sum works with any associative operator (internal/internalpipe/sum.go)
Each parallel chunk now seeds its accumulator from real data instead of the type's zero value, so Sum is correct for +, *, min, max, … — not just addition.
Range reports an exact length for floats (internal/internalpipe/constructor.go)
Len is now derived consistently with the value predicate, removing a float-rounding off-by-one at range boundaries (integers unchanged).
ff.Reduce forwards its optional seed (pkg/ff/reduce.go)
The initVal argument is now passed through to the underlying reducer, so seeded folds behave as documented.
genTickets is robust for non-positive input (internal/internalpipe/pipe.go)
Reordered the clamp so the token helper can't block on a zero/negative count.
Concurrency & robustness
Any() terminates promptly and respects known bounds (internal/internalpipe/any.go)
The parallel scan is now bounded and cancellable, so background workers stop as soon as a result is found, and Take-limited pipes are treated as bounded rather than "unknown length".
First() finishes deterministically for every pipe length (internal/internalpipe/first.go)
Chunk bookkeeping is now exact and the read window is clamped to the pipe length, so parallel First() completes for any length/worker combination and never reads past the end.
Yeti.Handle() is reentrancy-safe (internal/internalpipe/yeet.go)
Handlers now run outside the internal locks, so a handler can safely interact with the same Yeti (Yeet/Snag/Handle) without contention.
Documentation accuracy
Code doc comments match shipped API (pkg/pipe/constructors.go, pkg/pipies/filters.go)
Removed references to a helper that isn't shipped yet (Until) and a non-existent method (Pipe.Distinct()), and fixed a Fn typo — comments now describe the actual surface.
README examples compile and match behavior (README.md)
Corrected signatures and snippets (pointer returns for Any/First, uint16 parallelism, Take vs Gen results, reduce examples) so copy-paste examples work as written.
Tests
Added regression coverage for each area:

Sum with non-+ operators (*, min)
Any() goroutine-leak guard
First() with uneven chunking
Yeti handler reentrancy + concurrent Yeet/Snag/Handle
Range float length boundaries
genTickets with non-positive counts