fix: clamp negative Skip n to zero

[tr1v3r]

Problem

Skip(n) documents "n <= 0 keeps everything" (export.go), and element-wise it does — but the sizeHint arithmetic max(sizeHint-n, 0) treated a negative n as skipping a negative amount, inflating the hint:

stream.SliceOf(1, 2, 3).Skip(-1).Count()   // → 4  (wrong; actual elements: 3)
stream.SliceOf(1, 2, 3).Skip(-1).ToSlice() // → [1 2 3]

Count() short-circuits on the known hint, so it disagreed with the materialized result (audit finding M3 in the audit report / F3 in the core audit). ReduceBy's capacity hint inherits the same skew.

Root cause

No input clamp: the "keeps everything" contract was implemented by the iteration loop (nothing skipped) but not by the hint math.

Fix

Clamp at entry — if n < 0 { n = 0 } — so both the loop and sizeHint arithmetic agree: all elements kept, hint exact, Count() == len(ToSlice()).

Test evidence

New TestOps_SkipNonPositiveKeepsAll covers:

• Skip(-5)/Skip(-1)/Skip(0) on a known-size stream: Count() and ToSlice() agree (all 3 elements);
• unknown-hint path (From(seq, -1).Skip(-1)): same agreement;
• composed Skip(1).Skip(-2): hint arithmetic stays exact (Count == 3 of 4).

go build / go vet clean, go test ./... -race -count=1 all ok, golangci-lint 0 issues. Independent of #18/#19/#20 (base: master).