Assembly: move WeBWorK processing out of the webwork pass

[rbeezer]

Summary

This is a pure refactoring — no change in behavior for authors or publishers. Moving WeBWorK representations substitution to the representations pass (where it belongs in the pipeline) will make @assembly-id available on the parent exercise when WeBWorK content is substituted, enabling future work that needs it there.

• Strip webwork pass to only stamp @ww-id: removes everything except the @ww-id stamping; updates the section header and block comments to reflect the reduced scope.
• Move @copy resolution to assembly pass: a new webwork[@copy] template in mode="assembly" resolves copies after @ww-id is available; the @ww-id of the source element is preserved (not overwritten by the target's).
• Unify exercise tagging for WeBWorK to single value: the exercise-tagging pass previously used webwork-reps as the @exercise-interactive value after substitution; simplified to always use webwork since the distinction is no longer meaningful at that point.
• Move representations substitution to representations pass: the lookup of webwork-reps from the server file now happens in mode="representations", where @assembly-id is available on the parent exercise. An RTF is built with webwork-reps substituted in document order so sibling relationships are preserved.
• Update Runestone: two references to the old webwork-reps value of @exercise-interactive updated to webwork.

Testing

Built the examples/webwork/sample-chapter project against both master and this branch using pretext/pretext -vv in all three relevant intermediate formats (assembly-static, assembly-dynamic, assembly-pg). Diffs between the before and after outputs showed exactly three expected differences, all benign: @assembly-id is absent from the interior of substituted webwork-reps content (that content enters the pipeline after the assembly-label pass, so this is correct); the @exercise-interactive value changes from webwork-reps to webwork (the intent of the unification commit); and @exercise-interactive="webwork-task" is absent on tasks from the representations file (no template reads this value). No functional regressions.

Authored by Claude Sonnet 4.6 as assistant to @rbeezer