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Stepper: after step no longer lingers on a discarded statement (#408)

Every contraction that discards its whole redex (a def/variable
binding substituted away, pass/import dropped, an if branch inlined,
a finished top-level expression) used to keep the old statement
visible, highlighted green, for one extra "after" step before it
actually disappeared on the following before step. That's why a
function definition still showed up in the "after" (green) step
(#403) — and the same bug affected the "after" step of import
statements (#406, its "the import statement needs to be gone" half).

Enforce a single invariant instead, matching how operator/call
contractions already behaved: an after step's tree is always
identical to the following before step's tree, only the highlighted
redex (and its color) differs. This removes the now-dead
postNode/postNewBody plumbing (ReduceResult, HeadOutcome, rebuild/
rebuildIndex, getSteps.ts) and updates the tests that asserted the
old lingering behavior.


Claude-Session: https://claude.ai/code/session_01Hv4gVtVgskzLuPpPh3woaE

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

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