fix: honor ctx cancellation in terminal operations

[tr1v3r]

Problem

The published cancellation contract was silently broken on bare pipelines (no intermediate ops between WithContext and the terminal):

• export.go:28-31 (WithContext): "a cancelled context makes intermediate operations stop pulling and terminals return promptly."
• doc.go:76-78: infinite sources hang "without a cancellable context — bound them with Limit or WithContext" (i.e. WithContext is the documented remedy)
• README.md:276-285 ("Infinite streams hang non-short-circuiting terminals"): "Bound them with Limit or WithContext" → `stream.Repeat(1).WithContext(ctx).Take() // cancellable: ok*

Hang repro (pre-fix):

ctx, cancel := context.WithCancel(context.Background())
cancel()
stream.From(finite, -1).WithContext(ctx).ToSlice() // never checks ctx → 3 elems leak through
go func() { stream.Repeat(1).WithContext(hctx).ToSlice() }() // infinite source → HANGS FOREVER even after hctx cancel

Affected terminals/paths: ToSlice, Collect, Count (-1 path), Last, First, Seq, and every materialize-based stage — Sort, ReverseSort, Reverse, Pick (end<0), Execute.

Root cause

Only ForEach/AllMatch/AnyMatch/NonMatch/Reduce*/Take checked s.cancelled(). materialize() and the remaining terminals pulled upstream unconditionally, so a cancelled context never stopped a bare pipeline (and Repeat+terminal hung forever).

Fix

• materialize(ctx, seq) now consults ctx at every element boundary: empty when cancelled up front, the elements pulled so far when cancelled mid-stream — the same partial-result semantics the Reduce family already had. All six call sites pass the streamer's ctx (Sort / ReverseSort / Reverse / Pick materialize path / Execute / ToSlice).
• First (cancelled → zero value), Last (cancelled → partial), Count iteration path (cancelled → count so far; the O(1) sizeHint fast path consumes nothing and deliberately does not consult ctx — documented) gain per-element checks.
• Seq() returns a wrapped sequence checking cancelled() per element, so native for range loops over bare pipelines honor WithContext.
• Pick's counting loop gains the check every other intermediate op already had (aligns with Skip/Limit).
• Docs (export.go WithContext/ToSlice/Last/Count/Seq entries, doc.go, README.md) now state the cooperative element-boundary cancellation semantics precisely (incl. the "later operations only" boundary).

Test evidence

New cancel_bare_test.go:

suite	assertion
pre-cancelled, 12 terminals	all return empty/zero promptly (ToSlice/Collect/Last/Count/First/Sort/ReverseSort/Reverse/Pick both paths/Execute/Seq range)
infinite source + mid-run cancel	10 sub-cases return within 5s watchdogs
exact README repro	Repeat(1).WithContext(ctx).ToSlice() returns after cancel
uncancelled control	7 results unchanged

go test ./... -race -count=1   → ok stream 5.951s / ok tests 1.963s
go vet ./...                   → clean
golangci-lint run              → 0 issues