The PR's tests spy on the dedup kwarg reaching build/build_merge. Add graph-level
assertions for what the kwarg does: dedup=False preserves a fuzzy near-duplicate pair
that dedup=True merges, and exact-id collisions still collapse to one node either way
(the structural invariant that makes the flag safe).
Adds --no-dedup (default off, so dedup stays on) to skip the fuzzy near-duplicate merge
pass on build and incremental merge, for operators who would rather keep distinct symbols
that fuzzy-matched than pay the merge. Exact-id uniqueness is unaffected (it is a graph
structural invariant, not a dedup responsibility), and the flag arms the #479 shrink
guard so a surprising node drop is refused loudly. Mutually exclusive with --dedup-llm.