fix: avoid shared error variable in cycle tag closure

[masabni]

Fixes #150

cycleTag builds its renderer closure once, at parse time. At tags/iteration_tags.go:65 the closure assigned the write's error with = rather than :=, so it wrote to the err declared by the compiler at line 45 instead of to a local:

stmt, err := expressions.ParseStatement(...)          // line 45 — declares err
...
return func(w io.Writer, ctx render.Context) error {
    ...
    _, err = io.WriteString(w, values[n%len(values)])  // line 65 — captured, not shadowed
    return err
}, nil

The closure is built once per parse and shared by every render, so concurrently rendering one parsed *Template races on that variable. Declaring the error locally fixes it.

Only site. Every other returned closure already declares a fresh err inside itself before assigning (iteration_tags.go:89, include_tag.go:45, render_tag.go:389,404, engine.go:71), so this is one fix rather than a sweep.

Behavior unchanged. err is only ever non-nil on the FRender-to-a-failing-writer path, and the cycle counters were already per-render — cycleMap is created in loopRenderer.render and published via forloop[".cycles"] — so rendered output is unaffected.

Test. TestTemplate_Render_cycle_race parses one template and renders it from 16 goroutines. It sits next to the existing TestTemplate_Parse_race and TestTemplate_Render_race, which cover concurrent parse and concurrent render of distinct templates; this closes the remaining gap of one shared template. The template is deliberately just {% for %} + {% cycle %} with empty bindings, so it doesn't touch the caches Config shares by design. On unfixed code it fails:

WARNING: DATA RACE
Write at 0x00c00011e8f0 by goroutine 9:
  github.com/osteele/liquid/tags.cycleTag.func1()
      tags/iteration_tags.go:65
Previous write at 0x00c00011e8f0 by goroutine 21:
  github.com/osteele/liquid/tags.cycleTag.func1()
      tags/iteration_tags.go:65
...
--- FAIL: TestTemplate_Render_cycle_race (0.02s)
    testing.go:1617: race detected during execution of test

Checklist

• I have read the contribution guidelines.
• make test passes.
• make lint passes (0 issues). make vet and make ci pass too.
• New and changed code is covered by tests.
• Performance improvements include benchmarks. — N/A, this is not a performance change.
• Changes match the documented (not just the implemented) behavior of Shopify. — N/A: a pure concurrency fix with no observable behavior change.