SIA v1.6.0 — the exports lane leaves the monolith

First module extraction (roadmap Phase 2). No behavior change: ~1,000 lines of graph projection, two-lane domain-edge inference, corpus_edges, and export_graph move from bin/sialib.py into the new bin/siagraph.py via the same bind/invoke façade siasenses uses. sialib.py drops from 517 KB to 477 KB (98.6% → ~91% of the 512 KB marketplace scan cap).

Chosen by measurement, not guess: four parallel extraction maps ran before any cut, and the exports lane scored decisively cleanest, so it led rather than the originally-listed thought-recovery cluster. A sia-runtime-v5 member set was added to all four runtime-digest routines and the installer file lists so a partial tree cannot validate as an older complete one. Verified by the 857-test suite and a façade-identity smoke (the GraphProjectionPending exception is one shared class across the module boundary). See docs/ARCHITECTURE.md for the revised extraction order and CHANGELOG for the full record.