Phase 1: drive the full anchoring once on a throwaway context whose `SharedLiteralVisits` collector records every literal def-home VISIT, then mark the genuinely-shared literals on the real context. Must run BEFORE the real `processExports`/prerender so `prerender` roots shared literals at their canonical home on the real pass. The throwaway context is fully independent (its own prelude/anchor caches) so the count never pollutes the real output.